The Method of Doubt and Its Purpose

Descartes begins by adopting the position of a global sceptic. He wants to doubt everything because he wants to get back to the foundations of all knowledge in order to build science up from principles that are absolutely certain. He needs to doubt all his previous beliefs because he suspects that some of them are false; if some are, all might be, and therefore all subsequent beliefs would by insecure. He thinks that knowledge has to be certain, indubitable. He sets the standards for knowledge very high - perhaps too high, certainly for everyday purposes, but he is interested in the foundations of scientific knowledge, so perhaps it is reasonable to set very high standards. However, he doesn't live up to those standards himself. Before long he is trusting any ideas that are "clear and distinct", which is not the same as ideas that are indubitable.

Also, Descartes seems to confuse certainty with conviction. Just because you have a strong conviction that a belief is true, it doesn't follow that it is certainly true. He thinks certainty can come from a kind of rational insight (clear and distinct ideas), but this is different from an idea being logically certain.

Descartes' doubt is universal. He is questioning all his beliefs, not individually but by questioning the principles they rest on. For example, any belief based on trusting perception is undermined by the first wave of doubt. It is hyperbolic: extreme, over-the-top, absurd (eg the Evil Demon) - but it has to be like this because it has to get into all the corners of his usual ways of thinking.He must not allow himself to slip back into his habitual attitudes.

First Wave: Doubting the Senses

In the past Descartes has been deceived by his senses. Distant things appear small, for example. However, such perceptual illusions don't undermine all perceptions, he says. They aren't usual (they wouldn't be illusions if they were), and they can often be explained. In fact, Descartes could perhaps have gone further. There are good reasons for doubting all our perceptions in so far as we cannot be sure that the actual world is as it appears to be to us (but this point concedes that there is an actual world distinct from our perceptions).

Second Wave: The Argument from Dreaming

Descartes is a man, so he sleeps and has dreams. Sometimes his dreams are ordinary, sometimes extraordinary. But "there are no conclusive signs by means of which one can distinguish clearly between being awake and being asleep". So, how can he be sure that what he takes to be the real world, when he is awake, isn't actually a dream? He could be dreaming that he is sitting by the fire in his dressing gown, holding a piece of paper.

One response to this is that genuine perception is more coherent than dreams are and that we can usually tell when we are awake and when dreaming; but of course we could simply be dreaming that we can tell the difference! Another response is that the concept of dreaming depends on the concept of real perception, just as the concept of fake money depends on there being genuine money. If everything was a dream, would we be able to talk about dreams and reality? Personally, I think we could, because we would be dreaming that there was a dream world (within the dream-reality) and a "real" world (also within the dream-reality), so I think we could still have versions of the two concepts. Also, Descartes doesn't really need to prove that everything is or could be a dream, only that we can't know when we are dreaming and when we are awake. It is the correct application of the concepts that he is sceptical about.

But dreaming doesn't undermine all Descartes' beliefs. Even in dreams a square has four sides, 2+3 = 5, etc. So he introduces his secret weapon.

The Third Wave: The Evil Demon

What if our minds were controlled by a powerful and malignant demon who wants to deceive us about everything? Then everything would be thrown into doubt. I couldn't be sure anything I experience or think is real. The demon controls all my experience, perceptions, thoughts. He could be deceiving me even about mathematics and geometry, and about apparent features of objects such as shape, location and extension.

So Descartes now doubts everything. But somehow he has to get out of this global scepticism in order to establish a firm foundation for knowledge, but how? He has dug himself a deep hole!At present he does not know a single thing.