Skepticism is the practice of withholding assent where the grounds are insufficient. It functions both as a philosophical position and as the standard pressure test applied to every claim in epistemology.

The ancient schools

Pyrrhonism, traced to Pyrrho and systematized by Sextus Empiricus, was a way of life rather than a doctrine. For every argument the Pyrrhonist produced an opposing argument of equal weight, suspended judgment, and reported that tranquillity followed — ataraxia. The school was careful never to assert that knowledge is impossible, since that would be a claim of the kind it declined to make.

Academic skepticism, which took over Plato's Academy, argued the stronger line that nothing can be known, and was attacked for the self-refutation that invites.

Descartes's method

Descartes used doubt as a tool rather than a conclusion. In the Meditations he set aside anything that could be doubted: the senses deceive, dreams are indistinguishable from waking, and a powerful deceiver could falsify even mathematics. What survived was that he was thinking, and therefore existed — the cogito. His aim was to rebuild knowledge on that foundation, though most readers have found the rebuilding less convincing than the demolition.

Modern forms

The brain-in-a-vat scenario updates the deceiver: if your experiences were produced artificially, everything would seem exactly as it does. Since nothing in experience distinguishes the cases, it is unclear what rules the scenario out.

Responses divide. Some argue that knowledge does not require ruling out every alternative, only the relevant ones. Others argue that such scenarios are self-undermining, because a word's meaning depends on causal contact with what it refers to. Skepticism is rarely defeated outright; it is used to force accounts of knowledge to state exactly what they demand.