Schools of Philosophy: The Complete Guide (2026)
A school of philosophy is a community of thinkers who share a method, a starting question, and a body of conclusions. The major schools of philosophy — Stoicism, empiricism, existentialism, Confucianism and dozens more — are how the discipline organises itself, each with its own founders, key texts and points of disagreement. This guide maps every major school you are likely to meet, Western and Eastern, ancient and contemporary, with one-line definitions, key thinkers and a concrete suggestion for where to start reading.
Index
- Browse schools and concepts by branch
- What is a school of philosophy?
- The four main branches of philosophy
- Major schools of Western philosophy
- Major schools of Eastern philosophy
- How philosophical schools relate to each other
- How to choose a philosophical school to study
- How to read philosophy as a beginner
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Frequently asked questions about schools of philosophy
- What are the main schools of philosophy?
- What is the oldest school of philosophy?
- What is the difference between Western and Eastern philosophy?
- Are any schools of philosophy still relevant today?
- What are the four main branches of philosophy?
- How many schools of philosophy are there?
- What is the easiest school of philosophy to start with?
- Can you follow more than one school of philosophy?
- Explore more philosophical schools
Browse schools and concepts by branch
Every school and concept on this site belongs to one or more branches of philosophy. These hubs gather the thinkers and ideas of each branch in one place:
- Metaphysics — what exists and how reality is structured
- Epistemology — knowledge, justification and belief
- Ethics — how we ought to live and act
- Logic — valid reasoning and argument
- Political philosophy — power, justice and the state
- Aesthetics — art, beauty and taste
- Philosophy of mind — consciousness and the mind–body relation
- Philosophy of science — what makes knowledge scientific
- Philosophy of language — meaning and reference
- Philosophy of religion — arguments about God, faith and evil
What is a school of philosophy?
A school of philosophy is a tradition of thought built around shared assumptions and methods. Members of a school may disagree on details, but they agree on what counts as a legitimate question and what a reasonable answer looks like. Stoics, for example, all accept that virtue is sufficient for happiness — they argue about almost everything else.
Three terms are often used loosely and worth separating:
- A branch is a domain of inquiry, like ethics or logic. Every school touches several branches.
- A school (or school of thought) is a position or a set of positions on those questions, often associated with a founding figure or a city — Stoicism, Epicureanism, the Vienna circle.
- A tradition is a longer, broader thread — Western philosophy, Indian philosophy, analytic philosophy — that contains many schools.
Some schools form around a person (Aristotelianism), some around a place (the Vienna circle), some around a method (analytic philosophy), and some around a single doctrine (utilitarianism). Most schools change shape over time. The Stoicism of Zeno of Citium in the 3rd century BC is recognisably the same project as the Stoicism of Marcus Aurelius four centuries later, but the priorities shift.
The list that follows is selective. Academic catalogues count between 25 and 60 major Western schools, plus a comparable number of Indian and East Asian schools. We have organised them into Western (ancient, medieval and early modern, modern and contemporary, ethical) and Eastern, with a closing section on how the schools relate to one another.
The four main branches of philosophy
Before listing schools, it helps to know the four branches every school deals with in some form. They are the questions, not the answers.
Metaphysics
Metaphysics asks what exists. What kinds of things are there — minds, numbers, possibilities, God? Are causes real? Is time something or nothing? When Aristotle treated form and matter as the joint constituents of every physical object, he produced a metaphysical theory called hylomorphism.
Epistemology
Epistemology asks what we can know and how. Is knowledge justified true belief? Can we know anything without observation? The parsimony principle (often called Occam's razor) is a methodological rule from epistemology: do not multiply entities beyond necessity.
Ethics
Ethics asks how we should act. Is morality about consequences, duties or character? When Jeremy Bentham argued that "the greatest happiness of the greatest number" is the foundation of morals — see Bentham's theory of utilitarianism — he produced an ethical theory, not a metaphysical one.
Logic
Logic studies the structure of valid argument. Aristotle's syllogistic dominated for two thousand years; modern symbolic logic, developed by Frege and Russell from the late 19th century, is what most contemporary philosophers use today. Without logic, the other branches have nothing to argue with.
Aesthetics and political philosophy
Two further branches deserve mention. Aesthetics asks what beauty and art are. Political philosophy asks what a just society looks like and what authority states can claim. Both blur into ethics on one side and into empirical disciplines on the other.
Most schools privilege one branch. British empiricism is primarily epistemological. Stoicism is primarily ethical. The Vienna circle was primarily about philosophy of language and science. Knowing which branch a school lives in is the fastest way to understand what it is trying to do.
Major schools of Western philosophy
Ancient Greek and Roman schools (6th c. BC – 3rd c. AD)
- Pre-Socratics: the first thinkers who broke with mythological explanation. Thales (water as origin), Democritus (atoms and the void), Zeno of Elea (his paradoxes against motion).
- Platonism: the school of Plato (c. 427–347 BC), built around the theory of forms — abstract universals are more real than physical particulars.
- Aristotelianism: Aristotle's reaction against his teacher Plato. Substance, form, the four causes, virtue ethics. Aristotelianism dominated medieval philosophy through translation into Arabic and then Latin.
- Stoicism: founded by Zeno of Citium around 300 BC. Virtue is sufficient for happiness; emotions are mistaken judgements; live according to nature. Later Roman Stoics — Epictetus, Seneca, Marcus Aurelius — produced practical handbooks still read today.
- Epicureanism: founded by Epicurus in 307 BC. Pleasure (specifically the absence of pain and disturbance) is the highest good. Materialism in metaphysics, hedonism in ethics.
- Cynicism: Diogenes of Sinope and Antisthenes. Reject convention, money and status; live according to nature in a more austere sense than the Stoics.
- Skepticism: two main varieties — Pyrrhonism (suspend judgement on everything; Sextus Empiricus) and Academic skepticism. A direct influence on David Hume and on modern epistemology.
Medieval and early modern schools (5th – 17th c.)
- Scholasticism: the medieval university tradition. Aquinas, Duns Scotus, William of Ockham. Synthesises Aristotle with Christian theology.
- Cartesian rationalism: René Descartes (1596–1650) and his successors Spinoza and Leibniz. Reason, not the senses, is the route to certain knowledge. Begin with what cannot be doubted. Leibniz's pre-established harmony belongs here.
- British empiricists: John Locke, George Berkeley, David Hume. Knowledge starts with experience; the mind at birth is a blank slate. Hume pushed the position to its sceptical conclusion.
- Continental rationalists: Descartes, Spinoza, Leibniz considered as a loose movement opposing the empiricists. Innate ideas, deductive method, system-building.
- Renaissance and early modern science: Francis Bacon (inductive method) and Pierre Gassendi (a Christian revival of Epicurean atomism) bridge the medieval and modern worlds.
Modern and contemporary schools (18th c. onward)
- Kantian critical philosophy: Immanuel Kant tried to reconcile rationalism and empiricism. The mind imposes structure (space, time, causation) on experience. Kant set the agenda for the next two centuries. The view that some knowledge is independent of experience is apriorism.
- German idealism: Fichte, Schelling, Hegel. Reality is fundamentally mental or spiritual. See idealism and objective idealism.
- Pragmatism: an American school. Charles Sanders Peirce, William James, John Dewey. The meaning of an idea lies in its practical consequences. Instrumentalism is a related view about scientific theories.
- Existentialism: 20th-century continental movement. Søren Kierkegaard is often called the father; Jean-Paul Sartre the popular face. Existence precedes essence; freedom and abandonment define the human condition. Nietzsche, Albert Camus and Heidegger are usually grouped here too, with caveats.
- Phenomenology: Edmund Husserl, then Heidegger and Merleau-Ponty. Describe consciousness as it appears, before any theory.
- Analytic philosophy: the dominant 20th-century Anglo-American tradition. Frege, Russell, the early Wittgenstein, the Vienna circle, Quine, Kripke. Argue carefully, prefer clarity over depth, take logic and language seriously. The later Wittgenstein's use theories of meaning changed the field again.
- Continental philosophy: a loose umbrella for what analytic philosophers usually do not do — phenomenology, existentialism, hermeneutics (Wilhelm Dilthey, Friedrich Schleiermacher), critical theory, post-structuralism.
- Logical positivism / logical empiricism: the Vienna circle (1924–1936) — Carnap, Schlick, Neurath. A statement is meaningful only if it can be empirically verified or is a tautology. The verifiability principle was its core thesis. The school was killed by its own internal critique but reshaped how philosophers think about science.
- Naturalism: a 20th-century stance, not a single school. Philosophy should be continuous with the natural sciences. Quine and Sellars are the canonical figures.
- Schopenhauerian voluntarism: Arthur Schopenhauer treats Will, not reason, as the underlying reality. See voluntarism.
- Perspectivism: Nietzsche's view that there are no facts, only interpretations.
- Anti-realisms about meaning and truth: positions like conventionalism and non-classical logical views like dialetheism (some contradictions are true) are 20th-century answers to old problems about logic and language.
- Objectivism: Ayn Rand's 20th-century synthesis of metaphysical realism, rational ethics and political libertarianism. Outside the academic mainstream but widely read.
Major ethical traditions
Ethics is a branch, but several views are coherent enough to be schools in their own right.
- Utilitarianism: maximise overall happiness or wellbeing. Bentham and Mill in the 19th century. The contemporary distinction between act utilitarianism and rule utilitarianism is central to applied ethics.
- Deontology: duties and rules, regardless of outcome. Kant's categorical imperative is the canonical example.
- Virtue ethics: Aristotle's view, revived in the 20th century by Anscombe and MacIntyre. The right action is what a virtuous person would do.
- Contractualism: morality as what rational agents could agree to. Hobbes, Rawls, Scanlon.
- Care ethics: 20th-century feminist tradition. Moral life is grounded in concrete relationships of care, not abstract rules.
Major schools of Eastern philosophy
Indian schools
Indian philosophy is traditionally divided into orthodox (āstika) and heterodox (nāstika) schools, depending on whether they accept the authority of the Vedas.
- Vedanta: the most influential orthodox school. Three sub-schools — non-dualist (Advaita, Shankara), qualified non-dualist (Vishishtadvaita, Ramanuja) and dualist (Dvaita, Madhva). Reality is Brahman; the self is in some sense identical with it.
- Sankhya and Yoga: sister orthodox schools. Sankhya is a dualist metaphysics; Yoga adds practical discipline.
- Nyaya and Vaisheshika: logic and atomistic metaphysics respectively. Sophisticated work on inference and category theory.
- Mimamsa: focused on the proper interpretation of Vedic ritual.
- Buddhism: heterodox. The Four Noble Truths and the Eightfold Path. Mahayana and Theravada are the two great branches; Madhyamaka (Nagarjuna) and Yogacara are influential schools within Mahayana.
- Jainism: heterodox. Strict non-violence (ahimsa), perspectivism about truth, an austere ethical practice.
- Charvaka: heterodox materialism — only what is perceived is real. Largely lost; reconstructed from critics.
Chinese schools
- Confucianism: founded by Confucius (551–479 BC). Ethics of social harmony, filial piety, ritual propriety, the cultivation of the junzi (exemplary person). The golden rule appears in the Analects centuries before any Western source.
- Taoism: Laozi and Zhuangzi. Live in accordance with the Tao, the unforced way of nature. Wu wei (effortless action) is the practical principle.
- Legalism: Han Feizi, 3rd century BC. Strict law, rewards and punishments, distrust of moral exhortation. The political philosophy that unified China under the Qin.
- Mohism: Mozi, 5th century BC. Universal love, consequentialist ethics avant la lettre, defence of the weak.
- Neo-Confucianism: Song dynasty synthesis (Zhu Xi, Wang Yangming) of Confucian ethics with metaphysics borrowed from Buddhism and Taoism.
Japanese and other East Asian traditions
- Zen: a Mahayana Buddhist school adapted in China (Chan) and refined in Japan. Emphasises seated meditation and direct experience over scripture.
- Pure Land Buddhism: devotional Mahayana, large in East Asia.
- Bushido and Confucian state philosophy: practical ethical traditions developed in Tokugawa Japan and Joseon Korea.
- Korean philosophy: the Neo-Confucian Four-Seven debate of the 16th century on the relationship between principle (li) and material force (qi).
It is worth saying clearly: lumping all of this under "Eastern philosophy" is a Western convenience, not an internal category. Indian, Chinese and Japanese philosophy are no more a single thing than "Western philosophy" is. Treat the labels as shelves, not theses.
How philosophical schools relate to each other
Schools rarely arise in isolation. Most are visible reactions to a previous school, or attempts to combine two earlier positions.
- Empiricism leads to pragmatism. Locke, Berkeley and Hume worked out the consequences of starting all knowledge from sensory experience. A century later, William James and Dewey took empiricism further: the meaning of an idea is its observable consequences. Pragmatism is empiricism with a verb.
- Kant generates German idealism. Kant said the mind imposes structure on experience but kept a noumenal world beyond it. Fichte and Hegel cut off the noumenal half: if the mind structures everything we can know, then reality just is mental. Schopenhauer's voluntarism is a variant — Will replacing Mind.
- German idealism breaks into phenomenology and existentialism. Husserl took the idealist insight that consciousness structures experience and turned it into a method: describe experience without theorising about it. Heidegger turned the method back toward the human situation; Sartre and Camus made it popular.
- Positivism is reborn as analytic philosophy. The Vienna circle's verifiability principle could not survive its own scrutiny, but its values — clarity, formal precision, respect for science — became the operating culture of analytic philosophy.
- Stoicism keeps coming back. Stoicism influenced Spinoza and parts of early modern ethics; it left the academy almost completely in the 19th century; it returned in the 21st century through cognitive behavioural therapy, which inherits its claim that emotions follow judgements.
- Eastern and Western philosophy converge in the 20th century. Schopenhauer read Hindu texts. Heidegger corresponded with Japanese Zen scholars. Comparative philosophy is now a recognised subfield with its own journals.
A useful exercise: take any school in the lists above and ask "what school was this a reaction to?" The answer is almost never "none". Philosophy is a long argument, not a series of monuments.
How to choose a philosophical school to study
Most readers want a starting point, not an encyclopedia. Pick the entry below that fits your interest, then follow the links.
- You want to live a calmer life. Start with Stoicism. It is short on metaphysics and long on practice. Read Epictetus's Enchiridion (about 25 pages) and then Marcus Aurelius's Meditations. Relevant pages: Stoicism, Epictetus, Marcus Aurelius.
- You want to think clearly about ethics. Start with utilitarianism versus virtue ethics as a first contrast. Read Mill's Utilitarianism (60 pages) and Aristotle's Nicomachean Ethics book I. Relevant pages: utilitarianism, act utilitarianism, Aristotle.
- You want to know what we can know. Start with British empiricism through Hume. Read selections from Hume's Enquiry Concerning Human Understanding; move on to Kant. Relevant pages: British empiricists, David Hume, Immanuel Kant.
- You want to think about meaning and language. Start with the Vienna circle and the early Wittgenstein, then later Wittgenstein's use theories. Relevant pages: Vienna circle, use theories of meaning.
- You want to know what existence is for. Start with existentialism, but read it in the order it was written, not in the order it is famous: Kierkegaard's Fear and Trembling, then Nietzsche, then Sartre's Existentialism Is a Humanism (a lecture, not a book). Relevant pages: existentialism, Friedrich Nietzsche, Jean-Paul Sartre.
- You want a non-Western entry point. Start with Confucius's Analects (a few hours' reading) and Laozi's Daodejing. Then, if appealing, the Bhagavad Gita and an anthology of the Pali Canon. Relevant pages: Confucianism, Confucius.
A general rule: do not start with the founder's most difficult book. Start with a short text by a follower or a popular work, then return to the founder once you know what you are looking for. Reading Kant's Critique of Pure Reason before knowing what problem Kant was solving is a recipe for giving up on philosophy.
How to read philosophy as a beginner
A few practical rules will save you years.
- Read with a problem in mind. Philosophical texts answer questions. If you do not know what question the text is answering, you will not understand the answer. Sometimes the question is in the title (Hume's Enquiry Concerning Human Understanding); sometimes it is hidden, and you have to look it up.
- Read short before long. A 30-page essay by a follower of a school will teach you what the founder spent 600 pages working out. Bertrand Russell's History of Western Philosophy is opinionated but lucid. The free entries at the Stanford Encyclopedia of Philosophy and the Internet Encyclopedia of Philosophy are professional and trustworthy.
- Read at least two views on every question. No school has the last word. Reading only utilitarians produces a caricature of ethics; the same is true of any other school.
- Take notes in your own words. If you cannot summarise an argument in two sentences without quoting, you have not understood it.
- Argue with the text in the margin. Mark passages you disagree with. Most of the time the disagreement is the most useful thing in the book.
If you want a single starting list, here are three short books that have launched more philosophy careers than anything else: Plato's Apology (a trial speech), Mill's Utilitarianism, and Wittgenstein's Philosophical Investigations, sections 1–80. Read them in any order, take notes, and come back to this guide for context.
For practical edition and translation guidance on the schools above — which translation of Marcus Aurelius to buy, where to start with Aristotle, why never to open Kant with the first Critique, which books on Buddhism to ignore — see our Reading Guides, including the entry-level best philosophy books for beginners hub and dedicated guides to Buddhism and to moral philosophy.
Frequently asked questions about schools of philosophy
What are the main schools of philosophy?
The most influential Western schools are Platonism, Aristotelianism, Stoicism, Epicureanism, Scholasticism, Cartesian rationalism, British empiricism, German idealism, pragmatism, phenomenology, existentialism, and analytic philosophy. The most influential non-Western traditions include Vedanta, Buddhism (Madhyamaka and Yogacara especially), Confucianism, Taoism, and Zen. These are starting points, not a complete list.
What is the oldest school of philosophy?
In the West, the Pre-Socratics — Thales of Miletus is conventionally dated around 624 BC — are the oldest. In India, the early Upanishads (8th–6th c. BC) and early Buddhism (5th c. BC) are roughly contemporary. The earliest Chinese schools — Confucianism, Taoism, Mohism and Legalism — emerged in the same Axial Age, between the 6th and 3rd centuries BC.
What is the difference between Western and Eastern philosophy?
Generalisations are dangerous, but two patterns are real. Western philosophy is more often built around adversarial argument and logical proof; many Eastern traditions place comparable weight on practice and personal transformation. Western philosophy has tended to separate metaphysics from ethics; many Eastern traditions integrate the two. These are tendencies, not laws — Stoicism and Epicureanism look very Eastern in their practical orientation, and Indian Nyaya logic is as adversarial as anything in Athens.
Are any schools of philosophy still relevant today?
Yes. Stoicism shapes contemporary cognitive behavioural therapy. Pragmatism is the operating philosophy of much American legal reasoning. Virtue ethics drives most academic ethics published since the 1980s. Phenomenology underwrites a sizeable strand of cognitive science and qualitative social research. Analytic philosophy is the dominant style in most English-speaking philosophy departments.
What are the four main branches of philosophy?
Metaphysics (what exists), epistemology (what we can know), ethics (how we should act), and logic (what counts as valid argument). Aesthetics (what beauty and art are) and political philosophy (what makes a society just) are usually added as a fifth and sixth.
How many schools of philosophy are there?
There is no fixed number. Conservative academic catalogues list 25 to 30 major Western schools and a comparable number of Indian and East Asian schools. Finer-grained classifications run to 60 or more. The exact count depends on whether you treat, for example, Spinozism as a school in its own right or as a branch of Cartesian rationalism.
What is the easiest school of philosophy to start with?
Stoicism. The texts are short, the prose is direct, and the practical advice is usable on day one. Epictetus's Enchiridion is about 12,000 words; Marcus Aurelius's Meditations are private notes, not a system. After Stoicism, virtue ethics through Aristotle's Nicomachean Ethics book I is the natural next step.
Can you follow more than one school of philosophy?
Yes — most professional philosophers do. The technical name is informed eclecticism: take the strongest argument on each question, regardless of which school it comes from. The danger is shallow eclecticism — treating philosophy like a buffet without engaging with the tensions between schools. The fix is to keep reading.
Explore more philosophical schools
Philosophy Professor publishes a dedicated entry for every major school of thought. Use the directory below to drill into the topics that interested you above. Pages with the highest current demand include act utilitarianism, idealism, hedonism, British empiricists, instrumentalism, conventionalism, Confucianism, and the parsimony principle.
For deeper academic context, the Stanford Encyclopedia of Philosophy and the Internet Encyclopedia of Philosophy remain the gold standard for free, peer-reviewed reference material.
Choose a school below to read its dedicated page:
Abandonment in Existentialism: Sartre's Doctrine of Radical Freedom
Abandonment, in Sartrean existentialism, is the human condition of being thrown into a world without God, fixed values or excuses for our choices.
Absence Paradox: How Can What Is Not There Have Effects?
The absence paradox asks how absences — non-events, missing causes, things that are not there — can figure in causal explanations of what does happen.
Absolutism in Philosophy: Moral, Political and Metaphysical Forms
Absolutism is any view that there are unconditional truths or values that hold without exception. A guide to moral, political and metaphysical absolutism.
Act Utilitarianism: Definition, Examples and Criticisms
Act utilitarianism judges every action by its expected consequences. A clear definition, worked examples, the act-versus-rule debate, and the standard criticisms.
Activism in Philosophy: From Pragmatism to Engaged Theory
Activism in philosophy is the doctrine that thought is essentially tied to action — that knowing is itself a form of doing rather than passive contemplation.
Agnosticism: The Position That God's Existence Is Unknown or Unknowable
Agnosticism, coined by T. H. Huxley in 1869, is the position that the existence of God is unknown or unknowable. A guide to its varieties and the principal arguments.
Altruism: Comte's Coinage and the Ethics of Concern for Others
Altruism, coined by Auguste Comte, is the ethical principle of acting for the good of others. A guide to its varieties, the egoism debate and evolutionary altruism.
Analytic / Synthetic Distinction: From Kant to Quine
The analytic/synthetic distinction separates truths fixed by meaning from truths about the world. A guide from Kant's first Critique to Quine's "Two Dogmas".
Analytic Philosophy: Frege, Russell, Wittgenstein and the 20th-Century Tradition
Analytic philosophy is the dominant Anglophone tradition focused on logical analysis and clarity. Frege, Russell, early and late Wittgenstein, ordinary language, Quine, Kripke and contemporary developments.
Animism: The Belief That All Things Have Souls or Spirits
Animism is the belief that natural objects, plants, animals and phenomena are inhabited by souls or spirits. A guide to its anthropological and philosophical uses.
Anomalous Monism: Donald Davidson's Theory of Mental Causation
Anomalous monism, defended by Donald Davidson in 1970, holds that mental events are physical events but resist strict psychophysical laws.
Anthropomorphism: Attributing Human Traits to God, Animals and Things
Anthropomorphism is the attribution of human characteristics to non-human beings — gods, animals, natural phenomena. A guide from Xenophanes to contemporary debate.
Anthroposophy: Rudolf Steiner's Spiritual Science Explained
Anthroposophy, founded by Rudolf Steiner in 1912, is a spiritual movement combining theosophical, Christian and Goethean elements. A guide to its core ideas.
Anti-Realism: Dummett's Challenge to Verification-Transcendent Truth
Anti-realism, defended by Michael Dummett, holds that truth cannot transcend our capacity to recognise it. A guide to the position and its key arguments.
Antinomianism: The Doctrine That Christians Are Free from Moral Law
Antinomianism is the theological doctrine that Christians are released from moral law by grace. A guide to its history from Paul to the English Civil War.
Apatheia: The Stoic Ideal of Freedom from Destructive Passion
The Stoic ideal of freedom from irrational, destructive passions — not indifference or apathy, but the replacement of pathological emotions with well-grounded rational feelings (eupatheiai). Explained via Chrysippus, Epictetus, Seneca, and Marcus Aurelius.
Apriorism: The Doctrine That Some Knowledge Is Independent of Experience
Apriorism is the doctrine that some knowledge is independent of experience. A guide from Plato through Kant to contemporary debates about a priori knowledge.
Aristotle's Four Causes: Material, Formal, Efficient and Final Explained
Aristotle's four causes — material, formal, efficient and final — are the four kinds of explanation he held a complete account of any thing must give.
Asceticism: Voluntary Self-Denial in Philosophy and Religion
Voluntary self-discipline and renunciation of bodily pleasures in pursuit of a spiritual, ethical, or philosophical goal — from the Greek Cynics and Stoics to Buddhist renunciation, Christian monasticism, and Schopenhauer.
Associationism: How Ideas Combine in the Empiricist Theory of Mind
Associationism is the empiricist theory that complex thoughts arise from the association of simple ideas. A guide from Locke and Hume to twentieth-century critics.
Atheism: Definitions, History and the Main Arguments Explained
Atheism is the position that there is no God or gods. A guide to the main definitions, the history from Greek antiquity to today, and the principal arguments.
Atomic Uniformity Principle: Keynes on Independent Causal Atoms
The atomic uniformity principle, formulated by J. M. Keynes, holds that the world consists of independent causal atoms whose effects combine without alteration.
Atomism: From Democritus to Modern Physics and Logical Atomism
Atomism is the metaphysical doctrine that reality consists of indivisible basic units. A guide from the Greek atomists to logical atomism and beyond.
Attitude Theories of Meaning: Speaker Attitudes and Linguistic Content
Attitude theories analyse the meaning of utterances in terms of the speaker's psychological attitudes — desires, beliefs, intentions — rather than purely truth-conditional content.
Axiom of Reducibility: Russell's Patch on the Ramified Theory of Types
The axiom of reducibility: any higher-order property reduces to a first-order one. Russell's ramified theory, the difficulties for analysis, Ramsey's removal.
Ayn Rand's Objectivism: Reason, Self-Interest and Capitalism
Ayn Rand's Objectivism is the integrated philosophical system she developed in The Fountainhead and Atlas Shrugged: objective reality, reason, rational self-interest, and laissez-faire capitalism.
Bayesianism: Probability as Degree of Belief and Bayes's Theorem
Bayesianism is the view that probability is degree of rational belief, updated by Bayes's theorem in light of evidence. A guide to its philosophy and applications.
Behaviorism: From Watson and Skinner to Philosophical Behaviourism
Behaviourism is the view that psychology should study behaviour, not inner mental states — and, in its philosophical form, that mental terms refer to behavioural dispositions.
Bentham's Utilitarianism: The Felicific Calculus and the Principle of Utility
Jeremy Bentham's utilitarianism takes the greatest happiness of the greatest number as the foundation of morals. A guide to the principle of utility and the felicific calculus.
Bivalence: The Principle That Every Statement Is True or False
The principle of bivalence holds that every meaningful proposition has exactly one of the two truth values true and false. Distinct from excluded middle.
Boo-Hurrah Theory: A Caricature of Emotivism in Ethics
The "boo-hurrah" theory is a caricature label for emotivism — the view that moral utterances express attitudes of approval or disapproval rather than describing facts.
British Empiricists: Locke, Berkeley, Hume and the English Tradition
The British empiricists — Locke, Berkeley, Hume — developed the systematic empiricist tradition that knowledge comes from experience. A guide to the school.
Buddhism: The Four Noble Truths, the Eightfold Path and the Major Schools
Buddhism is the philosophical and religious tradition founded by Siddhartha Gautama in northern India in the fifth century BC, organised around the diagnosis that human suffering arises from craving and the prescription of an eightfold path of ethical conduct, mental discipline and insight that leads to its cessation. The tradition divides into three major branches...
Bundle Theory: Hume's Account of Substance and the Self
Bundle theory analyses substances and selves as bundles of properties or perceptions, denying any underlying substratum. A guide from Hume to contemporary versions.
Buridan's Ass: The Paradox of Choice Between Equally Good Options
Buridan's ass is the medieval thought experiment of a donkey starving between two equal piles of hay, raising deep questions about freedom, indifference and rational choice.
Cartesianism: Descartes' System and the School That Followed
The philosophical system founded by René Descartes — methodic doubt, the cogito, substance dualism, mechanism, and the school of occasionalist followers he inspired.
Categorical Imperative: Kant's Supreme Moral Law Explained
The categorical imperative is Immanuel Kant's supreme principle of morality. A guide to its three formulations and the deontological ethics it grounds.
Category Mistake: Gilbert Ryle's Diagnosis of Confused Concepts
A category mistake, in Gilbert Ryle's analysis, is the error of treating something as if it belonged to a different logical category from the one it actually belongs to.
Causal Principle: Every Event Has a Cause
The causal principle holds that every event has a cause. A guide to its formulation, defenders from Hume to Kant, and the challenge from quantum mechanics.
Causal Realism: The View That Causation Is a Real Feature of the World
Causal realism holds that causation is a genuine, mind-independent feature of the world rather than a mere projection of the human mind onto regularities.
Causal Theories of Meaning: Naturalising Mental Content
Causal theories of meaning analyse the content of words and concepts in terms of causal-historical relations to the world. A guide to Dretske, Fodor and the literature.
Causal Theories: Analysing Knowledge, Meaning and Reference Causally
Causal theories analyse philosophical concepts — knowledge, meaning, reference, perception — in terms of underlying causal relations rather than internal description.
Causal Theory of Knowledge: Goldman's Solution to Gettier
The causal theory of knowledge, developed by Alvin Goldman in 1967, holds that knowing requires the right causal connection between belief and the fact believed.
Causal Theory of Memory: Martin and Deutscher on Remembering
The causal theory of memory, developed by Martin and Deutscher in 1966, holds that remembering an event requires the event to have caused the present memory.
Causal Theory of Names: Kripke on How Proper Names Refer
The causal theory of names, developed by Saul Kripke, holds that proper names refer in virtue of an initial baptism plus a causal-historical chain of usage.
Causal Theory of Perception: Grice on Seeing What Causes Your Experience
The causal theory of perception, defended by H. P. Grice, holds that to perceive an object is to have a perceptual experience appropriately caused by it.
Causal Theory of Reference: Kripke and Putnam on Names and Natural Kinds
The causal theory of reference, developed by Saul Kripke and Hilary Putnam, holds that names refer in virtue of an initial baptism plus a causal-historical chain of usage.
Classical Theory of Probability: Laplace's Principle of Indifference
The classical theory of probability, formulated by Laplace, defines probability as the ratio of favourable to equally possible cases.
Coherence Theory of Truth: Truth as Membership in a Coherent System
The coherence theory of truth holds that a proposition is true if it coheres with a system of other propositions. A guide from the British idealists to contemporary versions.
Compatibilism: Reconciling Free Will with Determinism
Compatibilism is the view that free will is compatible with determinism. A guide from Hobbes and Hume to Frankfurt and contemporary defenders.
Computational Psychology: The Mind as a Computational System
Computational psychology treats cognition as the rule-governed manipulation of internal representations — the framework underlying classical cognitive science.
Conceptualism: Universals as Mental Concepts, Not Real Forms or Mere Names
Conceptualism is the medieval and early modern doctrine that universals exist as concepts in the mind — neither as Platonic realities nor as mere names.
Confirmation Principle: How Evidence Supports Theories
The confirmation principle concerns how empirical evidence supports or undermines scientific theories. A guide to Hempel, Bayesian and bootstrap accounts.
Confucianism: The Ethical and Political Philosophy of Kongzi and His Heirs
Confucianism is the ethical and political tradition founded by Kongzi (Confucius) in the sixth century BCE. A guide to its key concepts and historical development.
Connectionism: Neural Networks and the Parallel Distributed Processing Programme
Connectionism models cognition as activity in networks of simple units passing activation between each other — the framework underlying neural networks and deep learning.
Connexive Implication: Aristotle's Thesis and Modern Relevance Logic
Connexive implication is a non-classical conditional that requires real connection between antecedent and consequent — banning "If not P then P" and Aristotle's thesis as guides.
Consequentialism: The Family of Outcome-Based Moral Theories
Consequentialism says rightness depends on outcomes. Utilitarianism, the demandingness and integrity objections, scalar consequentialism, contemporary refinements.
Consistent Empiricism: Schlick's Version of Logical Positivism
Consistent empiricism is the name Moritz Schlick gave to his refined logical positivism, which tied meaning strictly to verifiability while avoiding metaphysical commitments.
Constructivism: Mathematics, Ethics, Knowledge — Three Domains
Constructivism in mathematics (Brouwer), in ethics (Rawls, Korsgaard, Street) and in knowledge (Goodman, Putnam). Origins, varieties and contemporary debates.
Contextualism: Knowledge, Aesthetics, Ethics, Philosophy of Science
Contextualism makes knowledge attributions context-sensitive. DeRose, Cohen, the bank cases, scepticism, contextualism in aesthetics and ethics.
Continental Rationalists: Descartes, Spinoza, Leibniz and the 17th-Century Tradition
The continental rationalists — Descartes, Spinoza and Leibniz — argued that reason can deliver substantive truths about reality independently of sense experience.
Contractualism: From Hobbes to Rawls and Scanlon's Reasonable Rejection
Contractualism grounds morality and politics in agreement. Hobbes, Locke, Rousseau, Rawls's veil of ignorance, Scanlon's What We Owe to Each Other.
Convention T: Tarski's Adequacy Condition for Truth
Convention T is Tarski's criterion for any acceptable definition of truth: every instance of "S is true if and only if S" must be a consequence of the definition.
Conventionalism: Truth and Choice in Logic, Mathematics and Science
Conventionalism explains a priori truths and theory choice as products of human convention rather than discovery. Poincaré, Duhem, Quine, and the regress problem.
Correspondence (Relational) Theories of Meaning: Words and the World
Correspondence or relational theories of meaning hold that words mean what they do because of relations between language and the world — denotation, reference, and causal connections.
Correspondence Theory of Truth: Truth as Agreement with Reality
The correspondence theory holds that a statement is true when it corresponds to the way things are. From Aristotle through Russell and Tarski to contemporary truthmaker theory and the deflationary challenge.
Counterpart Theory: Lewis on Possible Worlds and Transworld Identity
Counterpart theory analyses modal claims via similarity to counterparts in other possible worlds. David Lewis, transworld identity puzzles, Kripke's reply.
Covering Law Model: Hempel and Oppenheim's DN Account of Explanation
The covering law model analyses scientific explanation as deduction from laws and conditions. Hempel-Oppenheim 1948, the DN model, problems and successors.
Craig's Theorem: Eliminating Theoretical Vocabulary in Science
Craig's theorem (1953) shows theoretical terms in science can be replaced by observational substitutes. The instrumentalist hope, the realist reply.
Creative Evolution: Bergson's Élan Vital and the Critique of Mechanism
Creative evolution is Bergson's 1907 alternative to Darwinian mechanism. The élan vital, mediation between selection and teleology, religious appeal, critics.
Critical Realism: Sellars, Bhaskar, and the Middle Way in Perception
Critical realism mediates between direct realism and idealism. R.W. Sellars, the American critical realists, Roy Bhaskar's later social-scientific version.
Cynicism: Diogenes, Antisthenes, the Doggy Philosophers Explained
Cynicism is the ancient Greek movement of Diogenes and Antisthenes. Self-sufficiency, rejection of conventions, contrast with Stoicism, eight centuries of influence.
De Facto and De Jure Theories of Meaning: Description vs Norm in Use
De facto theories of meaning describe how words are actually used; de jure theories specify rules for correct use. The distinction lies at the heart of use theories of meaning.
Deduction Theorem: When Inference Becomes Implication
The deduction theorem states that if Q can be inferred from P, then "If P then Q" is provable as a theorem — a key bridge between rules of inference and conditional theorems.
Deductivism: Popper Against Induction in Scientific Method
Deductivism holds science should dispense with induction in favour of deductive testing. Popper, falsificationism, hypothetico-deductive method, critics.
Degrees of Truth: Vagueness, Sorites, and Many-Valued Logic
Degrees of truth treat truth as a continuous scale. The sorites paradox, fuzzy logic, supervaluationism, objections from classical logic.
Deism: God the Watchmaker, From Herbert of Cherbury to Jefferson
Deism is the doctrine of a non-intervening creator-God knowable by reason alone. Origins, Herbert, Toland, Voltaire, the American founders, the decline.
Denotation and Connotation: Mill's Distinction in Logic and Semantics
Denotation and connotation: Mill's 1843 distinction between what a word refers to and the qualities it implies. Comparison with Frege's sense and reference.
Deontology: The Ethics of Duty, Rights and Moral Rules
Deontology evaluates actions by whether they conform to moral rules or duties, not by their consequences. Kant's categorical imperative, divine command theory, contractualism and modern threshold deontology.
Descriptive Theory of Names: Frege, Russell, Searle, and Kripke's Reply
The descriptive theory of names treats names as disguised descriptions. Frege, Russell, Searle's cluster theory, Kripke's modal and epistemic objections.
Descriptivism: Moral Cognitivism Against Prescriptivism and Emotivism
Descriptivism in metaethics treats moral utterances as descriptive statements with truth values. Hare's critique, naturalism, contemporary cognitivist defenders.
Determinism: The Doctrine That Every Event Is Causally Necessitated
Determinism holds that every event is the necessary consequence of prior states of the world plus the laws of nature. Hard determinism, compatibilism, libertarianism — and what physics and neuroscience suggest.
Dialectic: From Socratic Argument to Hegelian Synthesis
Dialectic is a method of inquiry through structured argument and the resolution of contradictions. From Plato's Socratic elenchus to Hegel's dialectical idealism and Marxist materialist dialectic.
Dialetheism: The Logic That Some Contradictions Are True
Dialetheism holds that some contradictions are literally true. Graham Priest's defence, the liar paradox, paraconsistent logic and the contemporary debate over the law of non-contradiction.
Doctrine of Double Effect: Aquinas, Catholic Ethics, the Trolley Cases
The doctrine of double effect distinguishes intended from foreseen evils. Aquinas, Catholic ethics, the bombing/terror bombing case, trolley problems.
Doctrine of Internal Relations: Bradley, Moore, Russell, Holism
The doctrine of internal relations: all relations are essential to their bearers. Bradley, Moore's reply, monistic idealism, contemporary holism.
Doctrine of the Mean: Aristotle's Virtue Between Two Extremes
Aristotle's doctrine of the mean defines virtue as a disposition lying in a mean between two extreme vices. Nicomachean Ethics, examples, contemporary debates.
Double Aspect Theory of Mind: Spinoza, MacKay, Two Sides of One Thing
The double aspect theory says mental and physical events are two aspects of one underlying reality. Spinoza, MacKay, contrast with neutral monism and identity.
Double Negation Principle: Classical and Intuitionistic Logic Explained
The double negation principle says P implies not-not-P and vice versa. Classical logic accepts both halves; intuitionism rejects the second. Brouwer's reasons.
Dualism: Mind, Body and the Two-Substance View of Reality
Dualism holds that mind and body are fundamentally different kinds of thing. Substance, property and predicate dualism — from Descartes and the interaction problem to Chalmers and contemporary defenders.
Eclecticism and Syncretism: Combining Philosophical Traditions
Eclecticism selects compatible doctrines from rival schools; syncretism claims they were never really opposed. The two strategies recur in late antiquity, the Renaissance, and modern philosophy.
Egocentric Predicament: Ralph Barton Perry's Challenge to Empiricism
The egocentric predicament, named by Ralph Barton Perry in 1910, is the puzzle that all knowledge seems to begin with mental representations — risking solipsism for any empiricist epistemology.
Egoism: The Philosophy of Self-Interest in Psychology and Ethics
Egoism comes in psychological, ethical and rational varieties. From Hobbes and Mandeville through Sidgwick to Ayn Rand's objectivism and the contemporary debate over altruism.
Eleaticism: Parmenides, Zeno and the One Unchanging Reality
Eleaticism — the school of Parmenides, Zeno and Melissus — argued that reality is one and unchanging and that change and plurality are illusions. The first systematic appeal to logic.
Emergence Theories: Wholes That Cannot Be Predicted from Parts
Emergence theories hold that some phenomena arise from lower levels but cannot be predicted from or fully explained by them — life, consciousness, and complex systems as paradigm cases.
Emotive Theory of Truth: Choosing Truth Criteria by Attitude
The emotive theory of truth holds that the criterion of truth we adopt depends on our emotions or attitudes — a parallel to emotivism in ethics with similar Frege-Geach problems.
Emotivism: The Theory That Moral Statements Express Feelings
Emotivism holds that moral statements do not describe facts but express attitudes and try to influence others. Ayer, Stevenson, the Frege-Geach problem and the rise of expressivism.
Empiricism: Locke, Hume and the Theory That Knowledge Comes from Experience
Empiricism is the theory that knowledge comes from sense experience, defended by Locke, Berkeley, Hume and Mill against rationalism. History, doctrines and modern forms.
Empiriocriticism: Mach, Avenarius and Positivism Between Comte and Vienna
Empiriocriticism is the version of positivism developed by Ernst Mach and Richard Avenarius, banning all hidden entities — metaphysical or scientific — between Comte and the Vienna Circle.
Epicureanism: The Philosophy of Pleasure, Atoms and Tranquillity
Epicureanism is the school founded by Epicurus in Athens around 307 BCE, combining atomist physics, hedonist ethics and the pursuit of ataraxia. Lucretius, the Garden, the tetrapharmakos and the modern revival.
Epiphenomenalism: The View That Mental Events Have No Causal Powers
Epiphenomenalism holds that physical events cause mental events, but mental events cause nothing. Huxley's automaton thesis, Jackson's qualia argument, the causal exclusion problem and contemporary debates.
Epistemic Closure Principle: Knowledge, Skepticism, Dretske, Nozick
The epistemic closure principle says knowledge transmits across known entailment. Dretske and Nozick deny it to block skepticism; contextualists agree.
Essentialism: The Doctrine That Things Have Essential Properties
Essentialism holds that some properties are essential to a thing — necessary for it to be what it is. From Aristotle to Kripke and Putnam's necessity of origin, plus modern debates over natural kinds, identity and gender.
Existentialism: Kierkegaard, Sartre and the Philosophy of Radical Freedom
Existentialism holds that existence precedes essence and that humans must create meaning through free choice. Kierkegaard, Heidegger, Sartre, de Beauvoir, Camus explained.
Extensionality Thesis: Reducing Intensions to Extensions in Logic
The extensionality thesis holds that intensional contexts can be reduced to extensional ones — favoured by logical atomists, positivists and Quine, who denied that any coherent intensional logic is possible.
Externalism: How the World Outside Shapes Mind, Meaning and Knowledge
Externalism holds that mental content, meaning, justification or knowledge depend partly on factors outside the individual's head. Putnam's Twin Earth, Burge's social externalism, semantic externalism and externalist epistemology.
Fact–Value Distinction: From Hume's Law to the Modern Critique
The fact–value distinction holds that descriptive statements about what is the case cannot entail normative statements about what ought to be. Hume's law, Moore's open question, Putnam's collapse argument and contemporary debates.
Fallibilism: The Doctrine That No Belief Is Beyond Possible Revision
Fallibilism is the epistemological view that any belief, including the most confident, may turn out to be false. Peirce, Popper, Quine and the place of fallibilism in pragmatism, science and contemporary epistemology.
Falsificationism: Karl Popper's Demarcation of Science from Pseudo-Science
Falsificationism holds that scientific theories are those that forbid certain observations and can be refuted by them. Popper's demarcation criterion, the asymmetry of confirmation and falsification, criticisms by Duhem-Quine, Kuhn and Lakatos.
Fatalism: The Doctrine That What Will Happen Cannot Be Avoided
Fatalism holds that the future is fixed in such a way that human deliberation makes no difference. Aristotle's sea battle, theological fatalism, the idle argument and the contrast with determinism.
Fideism: Faith Above Reason in Tertullian, Pascal, Kierkegaard
Fideism holds that religious faith stands above or apart from reason. Tertullian, Pascal, Kierkegaard, Wittgensteinian fideism, criticisms.
Fido-Fido Theories: Direct-Object Theories of Meaning and Belief
Fido-Fido theories explain meaning or belief by direct relation to an object. Ryle's caricature, Schiffer's belief version, the Frege challenge, modern direct reference.
Finalism: Final Causes in Nature, Aristotle to Modern Teleology
Finalism is the view that natural phenomena have final causes. Aristotle's contribution, the modern critique, function and teleology in biology.
Finitism: Mathematics Without Completed Infinities, Hilbert and Beyond
Finitism restricts mathematics to finite constructions. Hilbert's programme, strict finitism, intuitionism contrast, the impact of Gödel.
Folk Psychology: Theory-Theory, Simulation, and Eliminativism
Folk psychology is the everyday explanation of behaviour by beliefs and desires. Theory-theory, simulation, eliminativism (Churchland) and Dennett's stance.
Formalism: Hilbert in Mathematics, Kant in Ethics, Greenberg in Art
Formalism in mathematics (Hilbert), ethics (deontology) and aesthetics (Bell, Greenberg). Origins, core claims, Gödel's incompleteness, and contemporary status.
Foundationalism: The Theory of Knowledge Built on Basic Beliefs
Foundationalism holds that knowledge rests on a foundation of basic, non-inferentially justified beliefs. Classical Cartesian foundationalism, modest contemporary versions and the regress argument.
Four Humors: Hippocrates, Galen and Ancient Greek Medicine
The four humors — blood, phlegm, black bile, yellow bile — were the basis of ancient Greek medicine and personality theory, surviving in modified form for nearly two millennia.
Frequency Theory of Probability: Von Mises, Reichenbach, Limits
The frequency theory defines probability as a long-run relative frequency. Venn, von Mises, Reichenbach, the reference-class problem and single-case difficulty.
Functionalism: The Theory That Mental States Are Defined by Their Causal Role
Functionalism holds that mental states are defined not by their physical realization but by the causal role they play between inputs, other mental states and behaviour. Putnam, Lewis, Armstrong and the multiple-realisability argument.
Goodman's Paradox: The New Riddle of Induction and Grue
Goodman's paradox uses the predicate "grue" to show that any body of evidence equally confirms incompatible hypotheses — the new riddle of induction.
Great Chain of Being: From Plato and Aristotle to Lovejoy
The great chain of being is the medieval and early modern conception of reality as a hierarchical ladder from God down through angels, humans, animals, plants and minerals.
Greatest Happiness Principle: Bentham, Mill and the Foundations of Utilitarianism
The greatest happiness principle, central to Bentham and Mill's utilitarianism, holds that we should maximise the happiness of the greatest number — the foundational slogan of consequentialist ethics.
Haecceitism: Thisness, Duns Scotus, and Modern Modal Metaphysics
Haecceitism: Duns Scotus's thisness, primitive individual identity, modern modal use, the dispute with Lewis's counterpart theory.
Hedonism: The Philosophy That Pleasure Is the Highest Good
Hedonism holds that pleasure is the only intrinsic good and pain the only intrinsic evil. Cyrenaic, Epicurean and modern utilitarian variants, plus the experience-machine objection.
Hedonistic Utilitarianism: Bentham, Mill, the Felicific Calculus
Hedonistic utilitarianism says we ought to maximise pleasure and minimise pain. Bentham's calculus, Mill's higher pleasures, the experience machine objection.
Hempel's Paradox: The Raven Paradox of Confirmation Explained
Hempel's paradox (the raven paradox) shows non-black non-ravens confirm "all ravens are black". Hempel, Goodman, Bayesian responses, contemporary status.
Hermeneutics: The Philosophy and Practice of Interpretation
Hermeneutics is the theory of interpretation. From biblical exegesis through Schleiermacher and Dilthey to Gadamer's philosophical hermeneutics and contemporary applications in law, social science and translation.
Historicism: Two Senses, From Dilthey's Verstehen to Popper's Critique
Historicism has two incompatible senses: the contextual understanding of Dilthey and the law-governed prophecy Popper attacked. Origins, development, critique.
Holism: The Doctrine That Wholes Are More Than the Sum of Their Parts
Holism is the family of philosophical views that wholes have properties not reducible to those of their parts. Smuts's coining, Quine and Davidson on confirmation and meaning, Gestalt psychology and contemporary philosophy of biology and physics.
Holistic Explanation: Peacocke, Davidson and the Whole-System View
Holistic explanation invokes whole interrelated systems of beliefs and desires rather than single factors. Peacocke, Davidson, perception and rational agency.
Human Nature: Aristotle to Hobbes, Marx to Evolutionary Psychology
Human nature is the question of whether humans share a fixed essence. Aristotle's rational animal, Hobbes, Rousseau, Marx, evolutionary psychology debates.
Humanity Principle: Grandy's Refinement of Davidson's Charity
The principle of humanity (Grandy 1973) refines Davidson's charity: interpret others as similar to ourselves in beliefs, desires and connections to reality.
Hume's Law: The Is-Ought Gap and Its Critics Explained
Hume's law says no ought follows from an is. The Treatise passage, Searle's counterexample, naturalistic fallacy, and contemporary defenders.
Hylomorphism: Aristotle's Matter and Form, Aquinas, Modern Revival
Hylomorphism is the doctrine that substance is composed of matter and form. Aristotle, Aquinas, modern revival in philosophy of mind and biology.
Hylozoism: Living Matter from Thales to Modern Panpsychism
Hylozoism is the doctrine that matter is intrinsically alive — defended by the earliest Greek philosophers and revived in different forms in the Renaissance and contemporary panpsychism.
Hypothetico-Deductive Method: Whewell, Popper, Conjecture and Test
The hypothetico-deductive method tests scientific hypotheses by deducing observable predictions. Whewell, Popper, falsification vs corroboration, modern critics.
Ideal Utilitarianism: G. E. Moore's Plural Goods, Bloomsbury
Ideal utilitarianism is G. E. Moore's pluralist version: aesthetic experience and personal relationships have intrinsic value beyond pleasure. Principia Ethica.
Idealism in Philosophy: Definitions, Types and Key Thinkers
Idealism is the philosophical view that reality is in some way mental. Its main forms — subjective, transcendental, absolute — and the philosophers who defended them.
Ideational Theories of Meaning: Locke, Aristotle, Words and Ideas
Ideational theories: words mean by standing for ideas in the mind. Aristotle, Locke, advantages over naming theories, Frege-Wittgenstein critique.
Identity Law: Aristotle, Bishop Butler, and the Third Law of Thought
The identity law: everything is what it is. Aristotle, Bishop Butler, Moore's Principia Ethica, the law's neglect compared with contradiction and excluded middle.
Identity of Indiscernibles: Leibniz, Black, the Symmetric Universe
The identity of indiscernibles says objects sharing all properties are identical. Leibniz, weak vs strong, Max Black's two spheres, the symmetric universe puzzle.
Identity Theory of Mind: Type vs Token Physicalism Explained
The identity theory of mind says mental states are identical with brain states. Type vs token versions, Smart and Place, multiple realisability objection.
Identity Theory of Predication: When "X is F" Means Identity
The identity theory of predication treats "X is F" as "X is identical with some F-thing" — an old, much-criticised analysis that conflates predication with identity.
Identity Theory of Truth: Bradley, McDowell, and True Thoughts
The identity theory of truth says a true thought is identical with the fact it represents. Bradley, Moore, McDowell. Defended against correspondence and coherence.
Ideology: From Destutt de Tracy to Marx, Mannheim and Althusser
Ideology runs from Destutt de Tracy's science of ideas to Marx's false consciousness, Mannheim's sociology of knowledge, and Althusser's interpellation.
Immaterialism: Berkeley's Doctrine That Only Minds and Ideas Exist
Immaterialism is George Berkeley's idealist doctrine that there is no mind-independent matter; reality consists only of minds and the ideas they perceive. The master argument, Three Dialogues, and the contemporary reception.
Impossibility of a Gambling System: Von Mises and Random Collectives
The principle of the impossibility of a gambling system: no rule selects winning subsequences from a random collective. Von Mises, place selection, randomness.
Improbabilism: Popper on Bold Hypotheses and Scientific Progress
Improbabilism, associated with Karl Popper, holds that scientists should seek the most improbable hypotheses — those that say the most and so are most easily refuted, but most significant if they survive.
Indeterminacy of Translation: Quine's Gavagai and the Limits of Meaning
Quine's indeterminacy of translation: rival manuals fit all evidence equally. Gavagai, ontological relativity, holism, doubts about synonymy.
Indeterminism: Quantum Chance, Free Will, and the Random-Action Worry
Indeterminism says some events have no cause. Quantum mechanics, libertarian free will, the worry that indeterminism reduces actions to randomness.
Indiscernibility of Identicals: Leibniz's Law and Substitutivity
The indiscernibility of identicals says identical things share all properties. Leibniz, Quine, the Cicero-Tully puzzle, opaque contexts and intensional logic.
Induction: The Logic of Reasoning from Particular to General
Induction is the form of reasoning that draws general conclusions from particular instances. Hume's problem of induction, Mill's methods, the new riddle of Goodman and Bayesian responses.
Inductivism: From Bacon to Mill, Carnap, and Modern Bayesianism
Inductivism holds that science proceeds by inductive generalisation. Bacon, Mill's methods, the problem of induction, modern Bayesian inheritance.
Infinite Divisibility: Zeno's Paradoxes, Atomism, the Continuum
Infinite divisibility: Zeno's paradoxes, the atomist alternative, Aristotle's potential infinity, modern continuum and quantum-mechanical limits.
Innate Ideas: Knowledge Born With the Mind
The doctrine of innate ideas holds that some concepts and knowledge are present in the mind from birth. Plato's anamnesis, Descartes's clear and distinct ideas, Locke's blank slate critique and the modern Chomskian revival.
Inscriptionism: Quine's Reduction of Mental Content to Particular Utterances
Inscriptionism reduces talk of beliefs and meanings to relations between speakers and particular utterances or inscriptions — a Quinean nominalist programme.
Instrumentalism: Scientific Theories as Useful Tools, Not Literal Truths
Instrumentalism holds that scientific theories are predictive tools rather than literal descriptions of unobservable reality. Mach, Duhem, Dewey and contemporary anti-realist successors.
Interactionism: The Doctrine That Mind and Body Causally Interact
Interactionism holds that mind and body causally affect each other. Cartesian interactionism, Princess Elisabeth's challenge, Eccles's neuroscience and contemporary defences against causal closure.
Internalism: The View That Justification Is Accessible to the Subject
Internalism in epistemology holds that justification depends only on factors accessible to the subject from within. Access internalism, mentalism, the new evil demon, and contrast with externalism.
Intuitionism: From Brouwer's Mathematics to Moore's Ethics
Intuitionism is the family of views that fundamental truths in mathematics or ethics are grasped by direct intuition. Brouwer's constructive mathematics, Moore's ethical intuitionism and Ross's prima facie duties.
Isolationism: Aesthetic Formalism and Political Non-Interference
Isolationism in aesthetics holds that art is self-sufficient, independent of context. Aesthetic formalism, contrast with contextualism, political senses.
Jourdain's Paradox: A Two-Card Variant of the Liar
Jourdain's paradox (1913) is a two-sentence variant of the liar paradox in which each half indirectly refers to the other, generating contradiction without explicit self-reference.
Justice: From Plato's Republic to Rawls and Contemporary Theory
Justice is the virtue of giving each what is due. Distributive, retributive, procedural and restorative kinds; from Plato and Aristotle to Rawls's veil of ignorance, Nozick's entitlement theory and Sen's capabilities approach.
Language of Thought: Fodor's Mentalese and Computational Mind
The language of thought hypothesis: thinking is computation in mentalese. Fodor 1975, Ockham's anticipation, productivity argument, contrast with connectionism.
Law of Continuity: Leibniz's "Nature Makes No Leaps"
Leibniz's law of continuity holds that nature makes no leaps — small differences in causes produce small differences in effects, a foundational principle of his metaphysics and calculus.
Law of Contradiction: One of the Three Laws of Thought
The law of contradiction holds that no proposition can be both true and false. From Aristotle to dialetheism, the principle anchors classical logic and tests its non-classical rivals.
Law of Excluded Middle: Either P or Not-P in Classical Logic
The law of excluded middle says that for any proposition P, either P or not-P is true. The principle defines classical logic and is rejected by intuitionism and dialetheism.
Lawyer Paradox: Protagoras and Euathlus, Self-Reference and Contracts
The lawyer paradox: Protagoras and his student Euathlus, the self-referential contract puzzle, modern resolutions and connection with the liar paradox.
Legal Positivism: Law as Social Fact, Not Moral Truth
Legal positivism holds that the existence and content of law depend on social facts, not on the moral merits of those laws. Bentham, Austin, Hart, Kelsen, and the Hart-Dworkin debate.
Legalism (Fajia): Law, Power, and the Chinese Legalist Tradition
The Chinese political philosophy (Fajia) that argued stable, powerful states require clear law, systematic reward and punishment, and concentrated sovereign power — from Shang Yang’s Qin reforms to Han Fei’s synthesis.
Legitimacy: The Right of Political Authorities to Govern
Political legitimacy concerns the right of authorities to rule and the duty of citizens to obey. From divine right and consent theory through Weber's three pure types to contemporary normative debates.
Leibniz's Law: The Two Principles of Identity Explained
Leibniz's law combines the indiscernibility of identicals and the identity of indiscernibles. Leibniz, contingent vs necessary readings, the Black ball-bearings puzzle.
Liberalism: Individual Liberty and Limited Government
Liberalism is the political tradition centred on individual liberty, equality before the law, and limited government — from Locke and Mill to Rawls and Berlin.
Libertarianism: Free Will and the Politics of Liberty
Libertarianism names two distinct philosophies: a political theory of minimal government and individual rights, and a metaphysical position on free will. Nozick, Friedman, Rothbard, Kane and the contemporary debate.
Linguistic Phenomenology: Austin and Ryle on Ordinary Language
Linguistic phenomenology is J. L. Austin's name for the careful, empirical analysis of ordinary language pursued by mid-century Oxford philosophy — distinct from Husserlian phenomenology.
Linguistic Philosophy: Austin, Ryle, and the Ordinary-Language Movement
Linguistic or ordinary-language philosophy: Austin, Ryle, Strawson, the post-war Oxford school, conceptual analysis, the decline in the 1960s.
Local Sign Theory: Lotze on How Sensations Get Their Place
Local sign theory, due to Rudolf Hermann Lotze, holds that we assign bodily location to sensations because each sensation carries a distinctive qualitative "local sign".
Logical Atomism: Russell, Wittgenstein and the Structure of Reality
Logical atomism, developed by Bertrand Russell (1918) and the early Wittgenstein (1921), holds that the world consists of independent atomic facts mirrored by atomic propositions in a logically perfect language.
Logical Empiricism: From the Vienna Circle to Postwar Philosophy of Science
Logical empiricism is the broader, postwar successor to logical positivism. Carnap, Hempel, Reichenbach and Feigl developed a more pluralistic account of confirmation, explanation and the unity of science.
Logical Positivism: The Vienna Circle and the Limits of Meaningful Discourse
Logical positivism, born in the Vienna Circle in the 1920s, holds that only statements verifiable by experience or true by definition are cognitively meaningful. Origins, doctrines, decline.
Logical Theory of Probability: Keynes, Carnap, and Confirmation
The logical theory of probability treats probability as a logical relation between hypothesis and evidence. Keynes 1921, Carnap, problems with the relation.
Logicism: The Reduction of Mathematics to Logic
Logicism is the philosophical thesis that mathematics is reducible to logic. From Frege's Grundgesetze and Russell-Whitehead's Principia Mathematica to Gödel's incompleteness and contemporary neo-logicism.
Manicheism: Mani's Cosmic Dualism, Augustine, Albigensians
Manicheism is the religion founded by Mani in 3rd-century Persia, a cosmic dualism of good and evil. Spread, Augustine's youth, decline, Albigensian inheritance.
Materialism: The Doctrine That Only Matter Exists
Materialism holds that everything that exists is matter or wholly dependent on matter. From ancient atomism — Democritus, Epicurus, Lucretius — to modern physicalism, dialectical materialism and contemporary philosophy of mind.
Mechanism: The Doctrine That Nature Operates by Material Causes Alone
Mechanism holds that natural phenomena are explained by the motions and impacts of material parts, without final causes or vital forces. From the Scientific Revolution through Descartes and Boyle to the contemporary debate over reduction.
Meinong's Jungle: Non-Existent Objects and the Theory of Sosein
Meinong's jungle: the realm of non-existent objects (golden mountain, round square). Meinong's distinction of being and existence, Russell's attack, modern revivals.
Meliorism: Progress, James, Dewey, and the Co-operating God
Meliorism is the doctrine that the world can be made better by human effort. Eliot, James, Dewey, theological meliorism, contrast with optimism and pessimism.
Mereology: The Formal Theory of Parts and Wholes
Mereology is the formal theory of parts, wholes and the relations between them. From Leśniewski's classical mereology to Lewis's gunky worlds, van Inwagen's Special Composition Question and contemporary metaphysics.
Metalanguage: Talking About Language at a Higher Level
A metalanguage is a language used to describe another language — its syntax, semantics, or pragmatics. The distinction is essential to logic, linguistics, and avoiding semantic paradox.
Methodological Theories: Holism, Individualism, Behaviourism Explained
Methodological theories prescribe a method without making substantive metaphysical claims. Methodological holism vs individualism, behaviourism, scepticism, solipsism.
Modal Realism: David Lewis and the Reality of Possible Worlds
Modal realism is David Lewis's bold thesis that all possible worlds are real concrete entities, just as real as our own. The argument from utility, ersatz alternatives and the contemporary modal-metaphysics debate.
Monism: The Doctrine That Reality Is Ultimately One
Monism is the metaphysical view that reality consists of a single substance, principle or whole. Parmenides, Spinoza, Hegel, neutral monism and Schaffer's priority monism.
Mysticism: The Pursuit of Direct Union with Ultimate Reality
Mysticism is the religious-philosophical tradition that seeks direct, non-discursive contact with ultimate reality. Plotinus, Eckhart, Sufism, Vedanta and the modern philosophical analysis of mystical experience.
Naive Realism: The View That We Perceive the World Directly
Naive realism holds that perception puts us in direct contact with mind-independent objects. The contemporary revival in disjunctivist philosophy of perception (Hinton, McDowell, Martin, Travis).
Naming Theories of Meaning: Reference, Russell, and the Fido-Fido View
Naming theories of meaning equate the meaning of a word with what it stands for. Mill, Russell, the Fido-Fido theory, and the objections that toppled it.
Nativism: The Cognitive Science of Innate Mental Structure
Nativism in cognitive science holds that the mind has substantive innate structure that shapes language, perception and reasoning. Chomsky's universal grammar, infant cognition, evolutionary psychology and the empiricist alternative.
Naturalised Epistemology: Quine's Reframing of the Theory of Knowledge
Naturalised epistemology, proposed by W. V. O. Quine in 1969, treats the study of knowledge as a chapter of empirical psychology. Replacement, cooperation and reliabilist versions, and the contemporary debate.
Naturalism: The View That Only Nature Exists and Science Is Continuous with Philosophy
Naturalism holds that reality consists of natural entities studied by science, and that philosophical method is continuous with scientific method. Ontological vs. methodological naturalism, Quine, Sellars, Dewey and the contemporary debate.
Necessitarianism: The Doctrine That Everything Must Be As It Is
Necessitarianism holds that the actual world is the only possible world; everything that exists exists necessarily. Spinoza's geometric theology, contemporary modal collapse arguments and the dispensability of contingency.
Negation Performative Theory: Denial as Speech Act
The negation performative theory analyses "not" as a speech act of denial rather than a semantic operator, raising Frege's classic objection about embedded contexts.
Negative Utilitarianism: Popper, Suffering-Focused Ethics, Critics
Negative utilitarianism prioritises the minimisation of suffering over the maximisation of happiness. Popper, Smart's painless killing objection, contemporary defenders.
Neo-Platonism: The One, the Intellect and the Soul in Plotinus and his Heirs
Neo-Platonism is the late-antique synthesis of Plato that emanates reality from the One through Intellect and Soul. Plotinus, Porphyry, Iamblichus, Proclus and the Christian, Islamic and Renaissance afterlives.
Neo-Pythagoreanism: Revival of Number Mysticism in Late Antiquity
Neo-Pythagoreanism revived Pythagorean number mysticism in the 1st century BC, blending arithmetic, Platonic forms, and metaphysical numerology that later shaped Neo-Platonism.
Neutral Monism: Mind and Matter as Aspects of One Underlying Stuff
Neutral monism holds that reality consists of one kind of stuff, neither mental nor physical, with mind and matter as different ways of organising it. Mach, James, Russell and the contemporary revival.
Nicod's Criterion: Confirmation by Positive Instances Only
Nicod's criterion: a generalisation 'all A are B' is confirmed by A&B, disconfirmed by A¬-B, with non-A irrelevant. Hempel's paradox dodged but at a cost.
Nihilism: The Denial of Meaning, Value or Knowledge
Nihilism denies the existence of meaning, value, knowledge or moral truth. From Russian nihilism and Nietzsche's diagnosis to contemporary metaphysical, moral and epistemological versions.
No-Ownership Theory of the Mind: States Without a Subject
The no-ownership theory of the mind holds that mental states exist without belonging to any owning subject — a view associated with bundle theories and Strawson's classic critique.
Nominalism: The Doctrine That Only Particulars Exist
Nominalism denies the existence of universals and abstract objects. From the medieval problem of universals — Roscelin, Abelard, Ockham — to contemporary trope theory, fictionalism and Quine's nominalist programme.
Non-Cognitivism: Moral Statements Are Not True or False
Non-cognitivism holds that moral utterances do not state facts and are not truth-apt. Emotivism, prescriptivism, expressivism and quasi-realism. Frege-Geach problem and contemporary debate.
Objective Idealism: Reality as Mind Beyond Individual Minds
Objective idealism holds that reality is fundamentally mental but not reducible to individual minds — defended by Hegel, Schelling, F. H. Bradley and the absolute idealists.
Objectivism (General): Mind-Independent Truth and Reality
Objectivism, in its general philosophical sense, holds that some domain contains objects, truths, or methods independent of human attitudes. Distinct from Ayn Rand's specific philosophy.
Occasionalism: Malebranche on God as the Only True Cause
Occasionalism, defended principally by Malebranche, holds that finite creatures are not genuine causes — God produces every event on the occasion of apparent natural causes.
Ockham's Razor: The Principle of Ontological Parsimony
Ockham's razor advises that entities should not be multiplied beyond necessity. From William of Ockham's medieval principle to its use in science, statistics, machine learning and contemporary metaphysics.
One Over Many Principle: Why Plato's Forms Exist
The one-over-many principle is the metaphysical motivation for Plato's theory of Forms: where many things share a property, there must be a single entity they share. The engine of Platonic universals.
Ontology: The Philosophical Study of What Exists
Ontology investigates what kinds of things exist and how they relate. From Parmenides and Aristotle to Quine's criterion of ontological commitment and contemporary meta-ontology.
Operationalism: Defining Concepts by Measurement Procedures
Operationalism, due to physicist Percy Bridgman, defines scientific concepts by the operations used to measure them — a strict empiricism that fragmented under its own consequences.
Organic Unities Principle: G. E. Moore on Wholes and Their Parts
The organic unities principle, central to G. E. Moore's Principia Ethica, holds that the value of a whole may differ from the sum of the values of its parts — wholes have emergent value.
Organicism: Wholes, Parts and the Living Analogy in Philosophy
Organicism treats wholes — societies, organisms, even the cosmos — as more than the sum of their parts, modelling reality on the integration of living bodies.
Origins of Life: Theories of How Living Cells First Came to Be
Theories of the origins of life — primordial soup, RNA world, mineral catalysis, hydrothermal vents — try to explain how biomolecules and living cells emerged from non-living matter.
Orphism: Mystery Religion, Reincarnation and the Roots of Platonism
Orphism is the strand of ancient Greek religious thought attributed to the mythical singer Orpheus, with doctrines of reincarnation and purification that shaped Pythagoreanism and Plato.
Panpsychism: The Theory That Consciousness Is Fundamental in Nature
Panpsychism holds that consciousness is a fundamental feature of physical reality. Constitutive, cosmopsychism, micropsychism, the combination problem and Galen Strawson explained.
Pantheism: God and the Universe Are One
Pantheism identifies God with the universe. Spinoza's Deus sive natura, classical and modern forms, contrast with panentheism and theism, and key arguments explained.
Paraconsistency: Reasoning Without Explosion in the Face of Contradiction
Paraconsistent logic allows reasoning to continue in the presence of contradictions without trivialising. Inconsistent theories can still be informative — the logical home of dialetheism.
Paradigmatism: Plato's Forms as Paradigms and the Third Man
Paradigmatism reads Plato's Forms as paradigms — perfect exemplars of which sensible objects are copies — a reading that powers the famous Third Man argument.
Parsimony Principle: Ockham's Razor and Simplicity in Philosophy
The principle of parsimony, also called Ockham's razor, holds that we should not multiply entities beyond necessity. Origins, formulations, applications and key objections explained.
Particularism: Why Some Philosophers Think Only Particulars Exist
Particularism holds that only particular things exist — not abstract universals. A guide to the doctrine, its variants and the debate with realism about universals.
Pascal's Wager Explained: Argument and Objections
Pascal's wager argues that betting on God's existence is rational because the potential gain is infinite and the loss is finite. Argument, decision matrix and objections.
Performative Theory of Truth: Strawson's Speech-Act Account Explained
The performative theory of truth, developed by P. F. Strawson in 1949, says that calling a statement true is performing the act of endorsing it.
Personalism: The Philosophy That Persons Are the Highest Reality
Personalism holds that the person is the central category of philosophy. Boston, French Catholic (Mounier, Maritain), Wojtyła and contemporary developments explained.
Perspective Realism: How Reality Looks Different from Different Standpoints
Perspective realism holds that objects have the properties they appear to have from each standpoint — without any single perspective being privileged.
Perspectivism: Nietzsche's Theory That There Are No Facts, Only Interpretations
Perspectivism is the view, associated with Nietzsche, that all knowledge is from a perspective and there are no neutral facts. Origins, contemporary forms and key objections explained.
Phenomenalism: The Theory That Physical Objects Are Bundles of Sense Data
Phenomenalism holds that statements about physical objects can be analysed into statements about actual and possible sense experience. Mill, Russell, Ayer and the failure of the project explained.
Phenomenology: Husserl, Heidegger and the Philosophy of Lived Experience
Phenomenology is the philosophical study of structures of consciousness as experienced from the first person. Husserl, Heidegger, Merleau-Ponty, Sartre and contemporary applications.
Philosophical Pessimism: Suffering and the Case Against Existence
The philosophical view that existence carries a net negative balance of value — that suffering outweighs satisfaction enough that non-existence would have been preferable.
Physicalism: The Theory That Everything Is Physical
Physicalism holds that everything that exists is physical or supervenes on the physical. Reductive and non-reductive forms, the mind-body problem and key objections explained.
Picture Theory of Meaning: Wittgenstein's Tractatus on Language and World
The picture theory of meaning, in Wittgenstein's Tractatus (1921), holds that meaningful sentences picture possible states of affairs in the world. Atomic facts, logical form and limits of language explained.
Plato's Theory of Forms: The Eternal Realities Behind Appearances
Plato's theory of Forms posits eternal, perfect realities of which physical things are imperfect copies. The Form of the Good, the Cave, third man argument and influence explained.
Platonism: The Theory of Forms and the Realm of Abstract Objects
Platonism holds that abstract objects, especially universals and mathematical entities, exist in a non-physical realm. Plato's Forms, mathematical platonism, and modern defences explained.
Plurality of Causes: Mill, Asymmetry, and Multiple Causal Routes
The plurality of causes: same effect can have different causes on different occasions. Mill, the asymmetry of explanation and prediction, vagueness of effects.
Positivism: Comte, the Vienna Circle and the Philosophy of Verifiable Knowledge
Positivism holds that genuine knowledge derives from sensory experience and verifiable observation. Auguste Comte, John Stuart Mill, the Vienna Circle and the verifiability principle.
Possible Worlds: Modal Logic, Necessity and the Ontology Debate
Possible worlds are complete, consistent alternatives to reality — the central tool philosophers use to analyse necessity, possibility and counterfactuals.
Pragmatic Theory of Truth: Peirce, James and the Long-Run Convergence Account
The pragmatic theory of truth analyses truth in terms of inquiry, success or convergence. Peirce's long-run, James's cash value, Dewey's warranted assertibility and key objections explained.
Pragmatism: Peirce, James, Dewey and the American Philosophical Tradition
Pragmatism judges ideas by their practical consequences. Founded by Peirce, popularised by James and Dewey, revived by Rorty and Putnam. History, doctrines and influence.
Pre-established Harmony: Leibniz's Doctrine of Mind, Body and the Monads
Pre-established harmony is Leibniz's solution to the mind-body problem: God arranges substances to act in coordinated ways without genuine causal interaction. Origins and influence explained.
Preference Utilitarianism: Singer, Hare and the Satisfaction of Preferences
Preference utilitarianism takes the good to be the satisfaction of considered preferences rather than pleasure. Singer, Hare, animal ethics and key objections explained.
Prescriptivism: R. M. Hare and the Logic of Moral Language
Prescriptivism is the metaethical view that moral judgements are universalisable prescriptions. R. M. Hare, the language of morals, universal prescriptivism and key objections explained.
Principle of Charity: Davidson's Methodological Constraint on Interpretation
The principle of charity, defended by Donald Davidson and Willard Quine, requires interpreters to maximise the truth and rationality of speakers' utterances.
Principle of Indifference: Equipossibility, Bertrand's Paradox, Symmetry
The principle of indifference: assign equal probabilities when there is no reason to prefer one outcome. Laplace, Bertrand's paradox, modern maximum entropy.
Principle of Individuation: What Makes One Thing Distinct from Another
The principle of individuation: what distinguishes one individual from another. Aristotle on matter, Scotus on haecceities, modern modal puzzles.
Principle of Limited Independent Variety: Keynes on Induction
Keynes's principle of limited independent variety: the qualities of a domain fall into finitely many independent groups. The Bayesian justification of induction.
Principle of Perfection: Leibniz on the Best of All Possible Worlds
The principle of perfection is Leibniz's claim that the actual world is the best of all possible worlds — God's optimal choice grounded in sufficient reason and divine wisdom.
Principle of Plenitude: Why a Perfect World Must Be Full
The principle of plenitude says a perfect universe must be as full as possible — every genuine possibility realised — a thesis traced from Plato to Lovejoy and modern modal metaphysics.
Principle of Sufficient Reason: Leibniz, Parmenides, Modern Debate
The principle of sufficient reason: nothing is without a reason. Leibniz, Parmenides, the cosmological argument, modern critics from Hume to Pruss.
Probabilism: Mild Scepticism, Confirmation, and the Catholic Doctrine
Probabilism: certainty unattainable so we settle for probability; positive confirmation in science; Catholic moral probabilism. Three distinct doctrines.
Process Philosophy: Whitehead, Bergson and the Metaphysics of Becoming
Process philosophy holds that reality is fundamentally processes rather than substances. Whitehead, Bergson, James, contemporary process metaphysics and theology explained.
Propensity Theory of Probability: Popper, Peirce, Single-Case Chance
The propensity theory says probabilities are dispositional features of experimental setups. Peirce, Popper, single-case chance, the reference-class problem.
Psychologism: Frege, Husserl, and the Critique of Mental Foundations
Psychologism: treating logic and meaning as psychological. Mill, the Frege-Husserl critique, the rise of analytic philosophy, modern naturalised epistemology.
Psychophysical Parallelism: Mind and Body in Two Causal Streams
Psychophysical parallelism holds that mental and physical events run in parallel causal series without interacting — a view shaped by Spinoza, Leibniz and the difficulties of dualism.
Pyrrhonism: Sextus Empiricus and the Ancient Skeptical Tradition
Pyrrhonism is the ancient skeptical school that suspends judgement on all dogmatic claims to achieve tranquillity. Pyrrho, Aenesidemus, Sextus Empiricus, the modes and ataraxia explained.
Pythagoreanism: The Mystical Mathematics of Pythagoras and His School
Pythagoreanism is the ancient school combining mathematics, music, cosmology and mystical religion. Pythagoras, the Pythagorean theorem, harmony of the spheres, transmigration explained.
Radical Empiricism: William James's Pragmatist Metaphysics
Radical empiricism: William James's name for his pragmatist metaphysics. Pure experience, neutral monism, the postulate of relations, contemporary descendants.
Radical Interpretation: Davidson on Meaning, Belief, and Charity
Radical interpretation: Davidson's project of constructing a theory of meaning for an alien language. Charity, humanity, conceptual schemes, contrast with Quine.
Ramified Theory of Types: Russell's Order-Stratified Solution to Paradox
The ramified theory of types adds orders within types to resolve both set-theoretic and semantic paradoxes — Russell's full solution in Principia Mathematica.
Range Theories of Probability: Laplace, Kneale, Infinite Alternatives
Range theories analyse probability as the proportion of favourable to total alternative outcomes. Laplace, Kneale, the problem of infinite ranges, contemporary status.
Rationalism: Descartes, Spinoza, Leibniz and the Power of Pure Reason
Rationalism is the philosophical theory that reason, not sense experience, is the primary source of substantive knowledge. Descartes, Spinoza, Leibniz, Kant and modern forms.
Real Self: Idealist Theory of Wants, the Real Will, Political Implications
The real self: the idealist theory that people's true wants differ from their expressed wants. Rousseau, Hegel, the political risks, contemporary autonomy debates.
Realism in Philosophy: Metaphysical, Scientific and Moral Realism Explained
Realism is the view that some entities or truths exist independently of minds. Metaphysical realism, scientific realism, moral realism and contemporary debates explained.
Reductionism (1): 19th-Century Mechanism in Biology Explained
Reductionism in 19th-century biology: Helmholtz, Ludwig, du Bois-Reymond, Brücke and the programme to reduce life to physics and chemistry. Vitalism rejected.
Reductionism (2): Theory Reduction, Nagel, Kim, Multiple Realisability
Reductionism in philosophy: Nagel's bridge laws, multiple realisability, Kim on functional reduction, ontological vs theoretical reduction explained.
Redundancy Theory of Truth: Ramsey, Strawson, Deflationism
The redundancy theory says calling a proposition true adds nothing. Ramsey 1927, Strawson, Quine's disquotation, Horwich's minimalism, contemporary deflationism.
Regularity Theory of Causation: Hume's Constant Conjunction Explained
The regularity theory of causation, originating with David Hume, analyses causation as nothing more than the constant conjunction of types of events.
Relativism: Cultural, Moral and Cognitive Relativism Explained
Relativism holds that truth, morality or knowledge is relative to a framework. Cultural, moral, cognitive and conceptual relativism, history from Protagoras to Rorty, key objections.
Relevance Logics: Anderson and Belnap, Strict Implication, Entailment
Relevance logics restrict entailment to relevant propositions. Anderson and Belnap, paradoxes of strict implication, paraconsistency, contemporary status.
Relevant Alternatives Theory: Dretske, Fallibilism, Skepticism
The relevant alternatives theory says knowledge requires ruling out only relevant alternatives. Dretske, the zebra example, contextualism, anti-skeptical work.
Reliabilism: Justification, Knowledge and Goldman's Reliable Process Theory
Reliabilism holds that a belief is justified if it is formed by a process that reliably produces true beliefs. A guide to Goldman's theory and the major objections.
Representationalism: Mental Images, Sense-Data and Indirect Perception
Representationalism holds that perception, memory and thought work via mental representations. A guide to the doctrine, its variants and the modern revival.
Resemblance Theories of Universals: Nominalism and Family Resemblance
Resemblance theories of universals: nominalism dispenses with universals via standard instances. Wittgenstein's family resemblance, the cluster theory.
Retributivism: Just Deserts in the Theory of Punishment
Retributivism: punishment is justified by the offender's just deserts, not by deterrence or reform. Kant, strong vs weak versions, contemporary debates.
Rule Utilitarianism: Hooker, Brandt and the Code That Maximises Welfare
Rule utilitarianism applies the greatest happiness principle to moral rules rather than individual acts. Brandt, Hooker, the contrast with act utilitarianism and key objections explained.
Semantic Atomism: Compositional Meaning from Word Atoms
Semantic atomism: sentence meaning is built from independently specifiable word meanings. Compositionality, contrast with semantic holism, Fodor's defence.
Semantics: The Philosophy and Science of Linguistic Meaning
Semantics studies linguistic meaning. From Frege's truth-conditions through Tarski, Davidson, Montague and dynamic semantics to contemporary formal and cognitive approaches.
Sensationalism: Knowledge from Sensations Alone
Sensationalism is the empiricist doctrine that all knowledge derives from raw sensations, with everything else built up by inference — defended by Mach, Condillac, and the early Vienna Circle.
Sense and Reference: Frege's Distinction in the Philosophy of Language
Gottlob Frege's distinction between sense (Sinn) and reference (Bedeutung) is the foundation of modern philosophy of language. The morning star puzzle, indirect reference and the contemporary debate.
Simple Theory of Types: Russell's Hierarchy Against Paradox
The simple theory of types stratifies classes into a hierarchy — objects, classes of objects, classes of classes — to block Russell's paradox without invoking ramification.
Situationism (Ethics): Joseph Fletcher and the Ethics of Love in Context
Situationism in ethics, defended by Joseph Fletcher, holds that moral duty cannot be reduced to rigid rules but must respond to each situation under the supreme principle of love.
Skepticism in Philosophy: Pyrrho, Descartes, Hume and Modern Forms
Philosophical skepticism doubts whether knowledge is possible. Ancient (Pyrrho, Sextus, Academic), Cartesian methodic doubt, Humean, modern epistemic skepticism explained.
Solipsism: The Theory That Only My Own Mind Exists
Solipsism holds that only one's own mind certainly exists. Metaphysical, epistemological and methodological solipsism, the problem of other minds and key responses explained.
Species Essentialism: The Natural-State Model from Aristotle to Linnaeus
Species essentialism holds that each species has a fixed natural state shared by all its members — an Aristotelian-Linnaean view rejected by modern population thinking and Darwinian biology.
Species Theory: What Is a Species and Why Does It Matter?
Species theory examines what defines a biological species — the biological, phylogenetic, ecological and recognition concepts that biologists and philosophers have proposed and defended.
Speciesism: The Ethics of Treating Other Species as Inferior
Speciesism is the doctrine — and the prejudice — that one species is innately superior to others, coined by Richard Ryder and central to Peter Singer's case for animal liberation.
Specious Present: How We Perceive Time as a Brief Stretched Now
The specious present is a short interval of time experienced as a single 'now'. A guide to E. R. Clay, William James, the cognitive science and the philosophical objections.
Speech Act Theory: How Words Do Things, from Austin to Searle
Speech act theory studies how saying something is also doing something. A guide to Austin's How to Do Things with Words, Searle's classification and the contemporary uses.
Stimulus-Response Model: The Engine of Behaviourist Psychology
The stimulus-response model treats behaviour as a function of environmental stimuli and learned responses, the foundational framework of Watson and Skinner's behaviourism.
Stoicism: Zeno, Epictetus, Marcus Aurelius and the Philosophy of Virtue
Stoicism is the ancient philosophy that virtue is the only good and external things indifferent. Zeno, Cleanthes, Chrysippus, Seneca, Epictetus, Marcus Aurelius, modern revival.
Subjective Idealism: Berkeley's "To Be Is to Be Perceived" Explained
Subjective idealism, defended by George Berkeley, holds that physical objects exist only as ideas in minds. Esse est percipi, the role of God, objections and influence explained.
Subjectivism: When Truth Depends on Human Beliefs and Attitudes
Subjectivism is the view that a given subject matter — ethics, taste, colour — depends on human beliefs and attitudes rather than on mind-independent facts.
Subjectivist Theories of Probability: De Finetti, Ramsey, Dutch Book
Subjectivist theories of probability analyse probability as degree of belief. Ramsey, de Finetti, the Dutch book argument, contemporary Bayesianism.
Swampman: Davidson's Thought Experiment Explained
The Swampman thought experiment, introduced by Donald Davidson in 1987, tests whether mental content depends on causal history. Setup, argument and replies.
Tacit Knowledge: Polanyi's Theory of What We Know but Cannot Tell
Tacit knowledge: Michael Polanyi's account of knowing more than we can articulate. Face recognition, scientific discovery, attention from and to.
Teleology: The Philosophy of Purpose, Function and Final Causes
Teleology investigates the role of ends, purposes and functions in nature and action. Aristotle's final causes, the mechanical revolution, Darwinian etiological functions and contemporary debates.
The Absolute in Philosophy: From Hegel to Bradley
The Absolute is the unconditioned, all-inclusive reality posited by post-Kantian idealism — Schelling, Hegel, F. H. Bradley — as the ground of finite experience.
The Five Ways: Aquinas's Proofs of God's Existence Explained
The Five Ways are Aquinas's five arguments for God's existence in the Summa Theologiae: motion, causation, contingency, perfection, and design.
The Golden Rule: Treat Others as You Would Have Them Treat You
The Golden Rule appears in nearly every major moral tradition. Confucian, Hebrew, Christian, Hindu, Buddhist and Islamic versions, plus modern formulations in Kant, Hare and contemporary metaethics.
The New Riddle of Induction: Goodman's Grue Paradox
Nelson Goodman's new riddle of induction shows that purely formal induction cannot distinguish projectible from unprojectible predicates. The grue paradox, entrenchment and contemporary responses.
The Private Language Argument: Wittgenstein's Investigations Explained
Wittgenstein's private language argument denies that anyone could have a meaningful language whose terms refer only to her own private sensations. The beetle in the box, S, criticisms.
The Socratic Method: How the Elenchus Works
The Socratic method uses systematic questioning to expose internal contradictions in an interlocutor's beliefs, leading to aporia or clearer thinking.
Theism: The Philosophy of Belief in a Personal God
Theism is the philosophical position that a personal God exists. Classical theism, the divine attributes, arguments for and against, and contrasts with deism, pantheism and atheism explained.
Theories of Meaning: A Map of the Field, From Frege to Wittgenstein
Theories of meaning map the main approaches: reference, sense, use, truth conditions, intentions, causal roles. Frege, Wittgenstein, Davidson, Grice.
Theory of Descriptions: Russell's 1905 Analysis of Denoting Phrases
Russell's theory of descriptions analyses denoting phrases like "the present king of France". On Denoting (1905), the present king of France, Strawson's reply.
Theory of Effluxes: Atomist Films, Perception and Dreams
The theory of effluxes — Empedocles, Democritus and Lucretius — holds that objects emit thin films that cause perception, dreams and thought. A key ancient theory of mind.
Theory of Speciation: How New Species Arise from Geographic Isolation
The theory of speciation explains how new species arise — chiefly through geographic isolation, divergence, and reproductive separation — as developed by Mayr, Dobzhansky and modern evolutionary biology.
Theory of Tropisms: Loeb's Mechanistic Account of Animal Behaviour
Jacques Loeb's theory of tropisms (c. 1912) treats all animal and human behaviour as forced reactions to physical stimuli, on the model of plant tropisms — a strict mechanistic biology.
Third Man Argument: Plato's Self-Critique of the Theory of Forms
The Third Man argument, raised in Plato's Parmenides, threatens the theory of Forms with infinite regress when self-predication and one-over-many are combined.
Three Laws of Thought: Identity, Non-Contradiction, Excluded Middle
The three laws of thought — identity, non-contradiction and excluded middle — have anchored Western logic since Aristotle. Their content, history and modern critics.
Trace Theory of Memory: Memory as the Storage and Retrieval of Mental Traces
The trace theory holds that memory consists in the storage of physical or mental traces formed at the time of experience and retrieved later. Plato's wax tablet, Aristotle, Hume, contemporary cognitive neuroscience and the rival reconstructive view.
Transcendental Idealism: Kant's Critical Philosophy of Space, Time and Knowledge
Transcendental idealism is Kant's doctrine that space, time and the categories are forms of the mind, not features of things in themselves. Phenomena, noumena and the Critique explained.
Trialism: Cottingham's Three-Substance Reading of Descartes
Trialism is John Cottingham's interpretation of Descartes as recognising mind, body, and a third union of mind and body characterised by sensation — beyond the standard dualist reading.
Tristram Shandy Paradox: Russell on Infinite Time and Autobiography
The Tristram Shandy paradox uses Sterne's slow-writing autobiographer to illustrate Bertrand Russell's puzzle about infinite series and the apparent paradoxes of completed infinity.
Truth Theory: Tarski's Semantic Conception of Truth
Tarski's semantic theory of truth (1933) gave the first rigorous formal definition of truth for formal languages, fixing the model for modern truth-conditional semantics.
Truth-Conditional Semantics: Frege, Tarski, Davidson Explained
Truth-conditional semantics treats the meaning of a sentence as its truth conditions. Frege, Tarski's Convention T, Davidson's programme, modern critics.
Truth-Conditional Semantics: Meaning as Conditions of Truth
Truth-conditional semantics, descending from Tarski and Davidson, identifies the meaning of a sentence with the conditions under which it is true — the dominant framework in contemporary formal semantics.
Uniformity of Nature Principle: Hume's Problem and Modern Refinements
The uniformity of nature principle: nature is uniform and the future resembles the past. Hume, Mill, Goodman, modern formulations and the problem of induction.
Universalism: Bentham's Slogan, Impartiality, Moral Universalism
Universalism in ethics is the impartial counting of everyone equally. Bentham's slogan, contrast with egoism and altruism, theological and political senses.
Universalizability: Kant, Hare, the Test of Moral Principles
Universalizability is the test that moral principles must apply to all relevantly similar cases. Kant, R.M. Hare, the trivialization objection, contemporary debates.
Use Theories of Meaning: Wittgenstein and Meaning as Use
Use theories of meaning hold that the meaning of a word is its use in language. Wittgenstein's slogan, language games, ordinary language philosophy, critics.
Utilitarianism: Greatest Happiness Principle, Variants and Objections
Utilitarianism judges actions by the consequences they produce, picking the option that maximises overall welfare. History from Bentham to Singer, variants, objections.
Utopianism: From More's Island to Modern Dystopia
Utopianism is the pursuit of perfect societies — from Plato's Republic and More's island to modern social blueprints — and its critique by 20th-century dystopian thought.
Verifiability Principle: Meaning, Logical Positivism, Ayer, Critique
The verifiability principle says a sentence is meaningful only if empirically verifiable or analytic. Vienna Circle, Ayer, weak vs strong, the self-refutation.
Vicious Circle Principle: Russell on Self-Reference and Paradox
The vicious circle principle, introduced by Russell and Whitehead, forbids defining an entity by reference to a totality to which it belongs — the philosophical motor of ramified type theory.
Vienna Circle: Schlick, Carnap, Neurath and Logical Positivism
The Vienna Circle (1924-1936) launched logical positivism. Schlick, Carnap, Neurath, Hempel; Wittgenstein on the fringes; dispersal under Nazism; legacy.
Virtue Ethics: Character, Eudaimonia and the Good Life
Virtue ethics holds that morality depends on cultivating good character rather than following rules or calculating outcomes — a tradition rooted in Aristotle and revived in the 20th century.
Vitalism: From Lebenskraft to Driesch and Bergson's Élan Vital
Vitalism holds that life cannot be reduced to physics and chemistry. Lebenskraft, Driesch entelechies, Bergson elan vital, defeat by molecular biology.
Voluntarism: Schopenhauer's Will, Theological Voluntarism, James
Voluntarism makes will the central reality. Theological voluntarism (Ockham), Schopenhauer's cosmic will, William James's will to believe, contemporary debates.
