we can be a stimulation or be dreaming.
Descates is trying to prove that we are not in a simulation or dream. He cannot prove it using observation or the senses, since any simulation or dream would mean that what we observe or what we sense is fake. That's the whole point of simulations, dreams and hallucinations: We cannot trust the senses.
So instead he tries to prove it using reason and logic alone. That is, he is using a rationalist Approach to prove a fact without having to resort to empirical data.
Here's another way of putting it:
The skeptic says: "It is possible that everything is an illusion"
Descartes responds: "I will show how that is not possible by proving the existence of something without referring to any sense data or observations, using reason alone."
But how do we know that the fact we are doubting something makes us real?
Descartes uses the following approach:
- I can doubt the existence of my body, I can never be certain that my body exists.
- I can however be certain that "doubt" is real, since the very act of doubting means that "doubt" happens.
- So we are now certain of the existence of doubt - but doubt is a type of thinking, so being certain of the existence of doubt means also that I am certain that I think.
- So even though it is possible that my body is an illusion, a trick, a dream, or a simulation, it is not possible in the case of my mind. I am certain that I am thinking and therefor I am certain that my mind is real - cogito, ergo sum - I think, therefore I am.
Why is the act of thinking given so much importance when it actually does not tell us any reality?
This is one of the main objections to Descartes reasoning: He says that thinking proves the reality of "I", that the mind is real and independent of the body. Several authors point out that all he proved is that "thinking" is real - but there's no reason why the reality of thinking implies the reality of "I", the thinking can be real, but the "I" can still be an illusion. See this response for more details.