Philosophical Thought Experiments
A thought experiment is a scenario you run in your head to test an idea — no laboratory required. Philosophers have used them for over two thousand years to isolate one question at a time: strip away every irrelevant detail, push a principle to its limit, and see whether your intuitions survive. This index collects the classic philosophical thought experiments. Each page explains the setup, what the scenario is meant to show, and where the argument has been attacked.
Epistemology and skepticism
Can you trust anything you believe? These three scenarios put knowledge itself on trial.
- Descartes' evil demon — a deceiver powerful enough to fake your entire world. The starting point of modern skepticism.
- Brain in a vat — the twentieth-century update: if your brain were wired to a simulation, could you ever tell?
- The Gettier problem — two short cases that broke the definition of knowledge philosophers had relied on since Plato.
Philosophy of mind
What is consciousness, and could a physical description of the brain ever capture it?
- Mary's room — a scientist who knows every physical fact about color but has never seen red. Does she learn something new when she does?
- The philosophical zombie — a being physically identical to you with no inner experience at all. If zombies are conceivable, is physicalism in trouble?
- Swampman — Donald Davidson's molecule-for-molecule duplicate, and what it says about meaning and memory.
Ethics
Utilitarianism promises a simple formula for right action. These two scenarios test it against intuitions most people refuse to give up.
- The utility monster — a creature that converts resources into happiness more efficiently than anyone else. Should it get everything?
- The experience machine — a machine that guarantees a lifetime of pleasurable experiences. Would you plug in — and if not, why not?
Political philosophy
- The veil of ignorance — John Rawls' device for choosing fair principles of justice: design society without knowing who you will be in it. Includes the original position.
Decision and rationality
What does it mean to choose rationally? Two puzzles that still divide decision theorists.
- Newcomb's paradox — one box or two? A predictor who is almost never wrong turns a simple choice into a fault line in decision theory.
- Buridan's ass — a donkey starves between two identical bales of hay. Can a perfectly rational agent fail to choose?
How thought experiments work
A good thought experiment is an argument in disguise. The scenario fixes every variable except the one in dispute, then asks what you would say. If your intuition contradicts the theory under test, something has to give: the theory, the intuition, or the setup itself. That third option matters — many of the best-known responses to these scenarios argue that the experiment smuggles in assumptions rather than testing them. Daniel Dennett called the persuasive ones "intuition pumps," a label that cuts both ways: they pump intuitions efficiently, but a pump can be rigged.
That is why each page here gives equal space to the objections. A thought experiment you only know one side of is not an argument — it is a slogan.
More scenarios on the way
This collection is growing. Coming next: the trolley problem, the ship of Theseus, the prisoner's dilemma, the Chinese room, and Nietzsche's eternal recurrence.
To see where these scenarios come from, browse our biographies of 162 philosophers and 325 schools and concepts, or see how rival positions stack up in our philosophy comparisons.
