BM asked:

How did Descartes prove res extensa basing from his argument cogito ergo sum?

Answer by Craig Skinner

Short answer: he didn’t succeed.

Descartes felt that a new grounding of knowledge was needed for science, the traditional Aristotelian grounding being unsound.

This sure knowledge to be reached by reason alone (not observation) from what remains certain after using the Method of Doubt.

The Method of Doubt is to accept as true only what is presented so clearly and distinctly to the mind as to be certain.

First, then, he must doubt everything learned through the senses (all empirical or a posteriori knowledge as we would say). For the senses can deceive, and also, at any moment, he can’t be certain he is not dreaming, or that his mind is not controlled by an ‘evil genius’ which deceives him about everything.

Secondly, he must doubt all truths of reason (rational or a priori knowledge). He feels that, even if dreaming, he knows that 2+3 = 5, but he considers that an evil genius could deceive him about mathematical truths, interfering with his thought every time he adds 2 and 3 so that he is sure (wrongly) that the sum is 5.

Having done all this doubting, the only certainty remaining, the Archimedean fixed point as he calls it, is ‘ I think therefore I am’ (the cogito).

Unfortunately his arguments back from the cogito to knowledge of a physical world of concrete things, including other people and his own body, are flawed (see details below).

The upshot is that one of his legacies is scepticism, rather than its resolution, and strong philosophical scepticism (about matter, the external world, causation and selves) later emerges, for example in the views of Berkeley and Hume.

His argument from the cogito can fairly be stated as follows:

1. I can’t doubt my existence as a thinking thing, so I know this (the cogito).

2. I know it solely by clear and distinct perception.

3. So, what I perceive very clearly and distinctly is true.

4. I have a clear and distinct idea of God.

5. The idea of God includes necessary existence, so God exists.

6. God, being all good, is no deceiver.

7. So, I can rely on my God-given reason and senses, properly used.

8. Using them I have clear and distinct ideas of extension, size, shape, situation, movement, duration and particulars of an external world.

9. So, the external world, including my body, exists.

Clearly we can challenge the argument at many points. I wont deal with all these challenges, but here are examples:

3. People have perceived all kinds of things clearly and distinctly but which are not true eg:

* sinners suffer eternally in hell.

* the Earth is flat.

* some people (say, women or some ethnic groups) are intellectually and morally inferior.

* flies/rats spontaneously generate in compost heaps/mud respectively.

5. Fine, IF God exists his existence is necessary, but this tells us nothing about WHETHER God does exist.

6. God (or the gods) could be jokers who get a kick out of deceiving us.

Of course Descartes didn’t really doubt that there is a world out there, or that he had a body. His scepticism is a ploy (methodological scepticism) to try to put his views on a rational footing.