Descartes and Scepticism

The main source for these arguments is Descartes's Meditations (Descartes 1641).

Descartes's sceptical argument has three stages: the Distance argument, the Dream argument, and the Demon argument.


We sometimes make mistakes about things that are far away or seen under difficult conditions. Therefore, our senses are fallible and we cannot trust them absolutely.

However, we might think that we can still rely on our senses when we see things in good light, and close to.


When we dream, we think that we see things in good light and close to, but we are wrong. Therefore, we cannot ever trust our senses: the external world might all be a dream.

However, even in dreams mathematical demonstrations still give the same results. Two plus two is always four. So maybe we can rely on mathematics and reasoning.


Suppose that there is an all-powerful demon who can deceive us in both our senses and our reasoning. Then we would make mistakes in logic and mathematics, while being convinced that we were right.

We can never produce any evidence that there isn't such a demon, so we cannot trust senses or reasoning.

The Cogito

I think, therefore I am. (In Latin, 'cogito, ergo sum', which is where the name of the argument comes from.)

Suppose that there is an omnipotent demon. If he is deceiving me, I exist. Even if he deceives me into thinking that I don't, I still do. Therefore, I can be sure that I exist, if I can be sure of nothing else.

This argument was first put foward about a thousand years before Descartes, by St Augustine in The City of God.

Clear and Distinct Ideas

Descartes goes on to claim that he knows the cogito because he perceives it clearly and distinctly. He also perceives that he has an idea of a perfect God, and that this is a clear and distinct idea. He has a clear and distinct idea that this idea could not have come from him. Therefore there is such a God. Such a God would not allow Descartes to be always deceived. Therefore he can trust his reasoning and, to a point, his senses.

This reasoning is often criticised as neither clear nor distinct.