That is, the mind is a thinking, non-extended entity and the body is non-thinking and extended.
His Discourse on Method and Meditations contain his important philosophical theories. The most important issues he noted were the threat of being deceived and the potential of being incorrect in his judgments, both of which would lead him into error.
That is a very bold statement and put in a seemingly simplistic way, recorded in his first meditations. Descartes believes that there is no way to be able to distinguish being in awake from being in a state of dreaming.
Having gone through her once-in-a-lifetime exercise in First Philosophy, where the degrees of reality and hierarchy of values are put in place and perspective, the reader or meditator is brought back to the stove to contemplate her own imperfect nature and how to amend it.
Yet there are serious doubts related to the treatise's major argument. His Discourse on Method and Meditations contain his important philosophical theories. His belief is that the mind is indivisible because it is not a physical thing.
From the start, Descartes ponders the certainty of any knowledge he holds, as well as the soundness of its source.