Descartes' cogito allows him to deny extreme sceptism; but then how does he actually make contact with the world as opposed to saying - I exist and the world is my (unconscious) invention -, i.e. solipsism?
To prove the existence of the res extensa, the sensible world, Descartes uses an argument that we nowadays don't accept anymore. First, he proves God:
- I have an idea of perfection
- An idea of perfection has to come from something perfect
- I am not perfect
- Then there has to be something else perfect (=God)
Then he says: if all that information from the senses isn't correct, I'm deceived, so there must be some bad deceiver. However, since God is ultimately good, he wouldn't accept such a bad deceiver, therefore I cannot be deceived. Therefore, the information from the senses is correct, and the sensible world exist.
On a metaphilosophical level, you could say Descartes breaks his world down and then builds it up again in his meditations:
- Assume the sensible world doesn't exist
- Assume God doesn't exist
- Assume you don't even exist yourself
- Cogito ergo sum: yes, I do exist
- Proof of God (via the idea of perfection)
- Assume God doesn't exist
- Proof of the existence of the res extensa
All these proofs are a bit outdated and there are criticisms on them. Especially the proof of God (and thereby also the proof of the res extensa) isn't accepted. However, in his time that was probably all very clear. The real problem he had, was the Mind-body problem (how can mind and body interact if they're separated?).
The problem was famously addressed by René Descartes in the 17th century, resulting in Cartesian dualism, and by pre-Aristotelian philosophers, in Avicennian philosophy, and in earlier Asian traditions. A variety of approaches have been proposed. Most are either dualist or monist. Dualism maintains a rigid distinction between the realms of mind and matter. Monism maintains that there is only one kind of stuff, and that mind and matter are both aspects of it.
In Cartesian dualism, there is contact between mind and body in the 'homunculus' (Latin for 'little man', but also the Latin name for the epiphysis), a little part of the brain1. The body sends information from the senses to the mind. The mind 'processes' those signals and returns commandos to the body. The mind can also do something without getting information from the senses, i.e. when it just thinks, and it can command the body in that case as well.
In this image, you can see the body-part of a human: eyes give information to the epiphysis, the mind processes the information, and sends a command to the arm, which moves.
René Descartes's illustration of dualism. Inputs are passed on by the sensory organs to the epiphysis in the brain and from there to the immaterial spirit.
1: Why there? According to Descartes that is the only part of the body that does not exist as two identical halves and doesn't have an identical twin. For example, it can't be the eye, because you have two, and it can't be your nose, because it has two nostrils.