Clearing the Ground
Descartes had long recognised that many beliefs absorbed in childhood had turned out to be wrong. Rather than correct them one by one — which would take a lifetime — he resolves to doubt everything at once and start again. The method is surgical: find the foundations, undermine them, and whatever is built on them falls automatically.
Whatever I have hitherto admitted as most true, that I received either from, or by my Senses; but these I have often found to deceive me, and ’tis prudence never certainly to trust those that I have (tho but once) deceived us.
Levels of Doubt
Descartes proceeds in stages. First, the senses: they have deceived him before, so they cannot be trusted. Second, the external world: what if he is dreaming? Even waking experience cannot rule this out. Third, mathematics: even two plus three equalling five might be wrong if an all-powerful God — or demon — has constructed him to be mistaken about the most obvious things.
Each level of doubt is more extreme than the last. The senses are unreliable in specific cases. The dream argument shows the whole of experience might be illusory. The demon hypothesis shows even reason itself might be corrupted.
Doubt as Method, Not Conclusion
This is not scepticism as a final position. Descartes is not claiming we can know nothing — he is deliberately clearing ground. The Meditations as a whole is the answer to the doubt that Meditation I opens. By the end, he has rebuilt knowledge on a foundation the doubt cannot reach: the thinking self, the existence of God, and the guarantee of clear and distinct ideas.
The method has ancient precedents in Pyrrho and the Academic Sceptics, and Augustine had used a version of the cogito to refute sceptics in the fifth century. But Descartes deploys doubt instrumentally — as a tool to find certainty — rather than as an end in itself.
