The method of doubt
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What if everything you believe were false — not most of it, but every last item, including that 2 + 3 = 5? Descartes dares himself to try it, on purpose, to see what survives.
The six Meditations on First Philosophy (1641) open with a demolition project. Descartes notices that over a lifetime he has accepted many falsehoods as if true, and that everything built on them is shaky. So he resolves, “once in the course of my life,” to raze the whole structure to the ground and rebuild it on foundations that cannot be shaken (Meditation I). The goal is not doubt for its own sake but a stable, indubitable foundation for the sciences.
His doubt is hyperbolic (exaggerated, pushed to the extreme) and methodological — a tool, not a sincere loss of conviction. The rule he adopts: treat as outright false whatever admits of even the slightest doubt, so that only the absolutely certain can pass. This is deliberately harsher than everyday reasonableness. A belief that is merely probably true gets thrown out with the false ones, because probability is not the bedrock he is hunting for.
Why is Descartes' doubt called methodological rather than sceptical?
The demolition comes in three escalating waves, each doubting a wider class of beliefs than the last. Wave one targets the senses; wave two targets the whole sensory world; wave three targets even reason and mathematics. Watch the doubt climb.
Wave 1 — the senses sometimes deceive. A tower looks round from afar and square up close; a straight oar looks bent in water. Since the senses have deceived me at least once, and “it is prudent never to trust completely those who have deceived us even once,” they cannot be a foundation (Meditation I). But note the limit of this wave: it only touches distant or unusual perceptions. Surely I cannot doubt that I am here, sitting by the fire, holding this paper?
Wave 2 — the dream argument. How do I know I am not dreaming right now? In dreams I have sat by fires, walked, and felt my own hands, all while lying asleep. And “there are no certain marks by which the waking state can be distinguished from sleep” (Meditation I) — no infallible test that tells me which I am in. So any belief grounded in present sense experience might be a dream’s fabrication. The entire external world, not just its distant edges, is now in doubt.
But the dream argument has a residue it cannot reach. Even a dream borrows its materials from somewhere. A painter may invent a mermaid, yet the colours are real; even the strangest dream is built from simpler elements — extension, shape, quantity, number, duration. So Descartes reasons that arithmetic and geometry might survive, since “whether I am awake or asleep, two and three added together are five” (Meditation I). The dream doubts composite, sensory things but seems to spare the simple natures and mathematics.
Wave 3 — the deceiving God / evil demon. To dislodge even mathematics, Descartes reaches for the most radical hypothesis. What if an all-powerful God made me such that I go wrong every time I add two and three? To keep this from feeling blasphemous, he restages it as a malicious demon (genius malignus), “supremely powerful and cunning,” who bends all his energy to deceive me — feigning the sky, the earth, my own body, and making even the simplest reasoning seem certain when it is false (Meditation I).
What does the evil-demon hypothesis add that the dream argument could not achieve on its own?
After three waves almost nothing is left upright: the senses, the external world, my own body, and even mathematics have all been provisionally set aside as if false. Descartes ends Meditation I not in despair but poised — he has emptied the ground completely so that whatever he finds next will be absolutely solid. The question that opens Meditation II: is there anything left at all that the demon cannot fake?