The Meditator reflects that he has often found himself to be mistaken with regard to matters that he formerly thought were certain, and resolves to sweep away all his preconceptions, rebuilding his knowledge from the ground up, and accepting as true only those claims which are absolutely certain.
All he had previously thought he knew came to him through the senses. Through a process of methodological doubt, he withdraws completely from the senses. At any moment he could be dreaming, or his senses could be deceived either by God or by some evil demon, so he concludes that he cannot trust his senses about anything.
Ultimately, however, he realizes that he cannot doubt his own existence. In order to doubt or to think, there must be someone doing the doubting or thinking. Deceived as he may be about other things, he cannot help but conclude that he exists. Since his existence follows from the fact that he is thinking, he concludes that he knows at least that he is a thing that thinks.
He further reasons that he comes to know this fact by means of his intellect, and that the mind is far better known to him than the body.