I Think, Therefore I Am: Descartes and the Search for Certainty

With Descartes, philosophy turns a corner, and you can feel it happening inside his Meditations on First Philosophy. Writing in the seventeenth century, Descartes was not content to inherit the wisdom of the ancients and the medievals. He wanted to start over, to tear the whole building of knowledge down to the foundations and rebuild only on what could not possibly be doubted. The result is one of the most thrilling and influential experiments in the history of thought, conducted entirely inside one man’s head over six days of meditation, and it gave us a sentence almost everyone knows: I think, therefore I am.

Here is what Descartes taught me about radical doubt, about the one certainty that survived it, and about the famous problem he left for everyone who came after.

Key takeaways

• Radical doubt: reject everything that can possibly be doubted to find an unshakable foundation.
• The senses can deceive, and even an evil demon might be feeding us a false world.
• “I think, therefore I am”: the act of doubting proves the existence of the doubter.
• The mind is known more certainly than the body, which sets up the mind-body problem.
• Modern philosophy begins here, with the self rather than the cosmos.

Tearing it all down

Descartes begins with a startling act of demolition. Realising how many of his beliefs, accumulated since childhood, had turned out to be false, he resolves to doubt everything, to treat as false anything that admits the slightest doubt, until he reaches something absolutely certain or proves there is nothing certain at all. He doubts the senses, which sometimes deceive us. He doubts the external world, since he cannot always tell waking from dreaming. He even imagines an all-powerful evil demon devoted to deceiving him about everything, including mathematics. It is a breathtaking willingness to suspect the whole of his experience, and it clears the ground completely.

The one thing that cannot be doubted

And then, at the bottom of all this doubt, Descartes finds his rock. However powerfully the demon deceives him, there is one thing the deception cannot touch: that he is thinking. To be deceived, he must exist to be deceived; to doubt, there must be a doubter. I think, therefore I am. Even if everything else is illusion, the existence of the mind doing the doubting is undeniable. There is something genuinely electrifying about this move, the discovery of certainty not in the outer world but in the bare fact of one’s own thinking. It is the same insight, I noticed, that Avicenna reached centuries earlier with his Floating Man, now made the cornerstone of an entire new philosophy.

The problem he handed to the future

Having secured the thinking self, Descartes concludes that the mind is better known, and more certainly real, than the body, and that the two are fundamentally different kinds of thing: the mind a thinking, non-physical substance, the body an extended, physical one. This is the famous Cartesian dualism, and it created a problem that has haunted philosophy ever since. If mind and body are completely different kinds of substance, how on earth do they interact? How does a thought move an arm? Descartes never solved this convincingly, and I find it fascinating that his most lasting legacy may be not his answer but his question. He gave modern philosophy both its method, systematic doubt in search of certainty, and one of its deepest unsolved puzzles.

What I’m taking with me

Reading the Meditations, I felt I was watching the modern mind being born, the turn inward, the demand for certainty, the placing of the thinking self at the centre of everything. What I am taking from Descartes is partly a method, the willingness to question my inherited certainties and ask what can really survive scrutiny, and partly a humility about its limits. His radical doubt secured one luminous certainty and then struggled to rebuild the world from it. That is the drama of the book: a brilliant mind discovering both the power and the loneliness of starting from “I think.”

Frequently asked questions

What is Descartes’ Meditations on First Philosophy about?

It is Descartes’ attempt to rebuild knowledge on absolutely certain foundations by doubting everything that can be doubted, arriving at the certainty of his own existence as a thinking thing.

What does “I think, therefore I am” mean?

That the very act of thinking or doubting proves the existence of the one who thinks. Even if everything else is doubtful, I cannot doubt that I exist while I am doing the doubting.

What is Cartesian dualism?

Descartes’ view that mind and body are two fundamentally different kinds of substance, a non-physical thinking mind and a physical extended body, which raises the still-debated question of how they interact.

References & further reading

1. Descartes, René. Meditations on First Philosophy. (1641; translated by John Cottingham, Cambridge University Press, 1986.)
2. Williams, Bernard. Descartes: The Project of Pure Enquiry. Penguin, 1978.
3. Cottingham, John. Descartes. Blackwell, 1986.