Skepticism

Skepticism is not the claim that nothing is true. It is the observation that our evidence is consistent with more than one story about the world, and the demand that we notice.^[1] The problem of the external world is the canonical form: every experience you could use as evidence that there is a world out there would be exactly the same if there weren't — a dream, a demon, a simulation. Nothing inside experience distinguishes the cases, which is precisely why realism and idealism can't be settled by looking harder.

The ancient version was a way of living rather than a thesis. Pyrrhonists set opposing arguments of equal weight against each other, suspended judgement, and reported that tranquility followed — and they were careful not to assert even that knowledge is impossible, which would be one more claim needing defence.^[2] Descartes used doubt the other way round: as a tool for finding a foundation that resists it.

What comes out of this is worth keeping even if you find the conclusion unliveable. Certainty is rarer than it feels; most of what we know rests on induction and testimony rather than proof; and "I can't rule that out" and "that's a serious possibility" are different claims, which is the distinction a good skeptic keeps and a bad one collapses.