The big idea: Imagine a basket of apples: to be sure none are rotten, you tip them all out and only put back the ones you've checked.
That's how René Descartes opens the Meditations. He decides to throw out every belief he can find any reason to doubt — not because he thinks it's all false, but to see what, if anything, survives.
This is the method of doubt: doubt is a tool, not a mood. Descartes doubts on purpose, to reach something he cannot doubt.
Hold onto this: Descartes isn't a sceptic who gives up on knowledge. He uses doubt to build knowledge — clear the shaky ground first, then rebuild on rock.
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He doesn't doubt at random — he doubts in three widening waves, each stronger than the one before.
The three waves: Wave 1 — the senses deceive. A straight stick looks bent in water; a tower looks round from far off. Something that has fooled you once could fool you again, so the senses aren't a safe foundation.
Wave 2 — the dream argument. Right now you feel sure you're awake. But dreams feel just as real from the inside, and you've been fooled before. So how do you know you aren't dreaming this? You can't be certain — so even ordinary 'I'm sitting here' beliefs wobble.
Wave 3 — the evil demon. Descartes imagines a powerful evil demon bent on deceiving him about everything — even that 2 + 3 = 5. If such a deceiver were possible, nothing at all would be safe.
Checkpoint — the method of doubt: In one line: doubt everything the senses, dreams or an evil demon could fake, and see what's left standing. Hold that — the next micro finds the one thing that survives.
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The doubt looks extreme, but it has a clear job to do.
Doubt as a filter: Descartes wants certainty — not beliefs that are merely likely, but ones that cannot be wrong. The evil demon is his hardest possible test: if a belief survives even a super-powered deceiver, it's certain. Anything that fails the test gets set aside, not thrown away forever — just until it's proven safe.
Go further — higher-level insight: The doubt is methodological, not real. Descartes doesn't genuinely believe a demon is fooling him — he pretends to, as a filter. Noticing that the doubt is a deliberate tool (not despair) is what separates a top answer from one that calls Descartes 'just a sceptic'.