Descartes claimed that there is an essential distinction between the mind (or soul) and body. Proceeding from his famous maxim, “I think, therefore I am,” Descartes concluded that the first thing I can be certain of is my own existence. Even if I doubt my own existence, the fact that there is an “I” to do the doubting proves that I exist. I can be certain of my internal self. But I can never be totally certain of things in the external world. I can’t fully trust my sense perceptions. This goes along with his separation of mind and body. I am certain that I have (am) a mind but I must always question the external.
Descartes supposed that a mind (self) inhabited a body, like a ghost in a machine, and interacted with the world in that way. To understand the external world or the universe, I must use my inner consciousness to contemplate clear and distinct ideas. If those clear and distinct ideas I think of correspond to things I observe in the external world, I can at least begin to believe they might be true.
He supplements this with his belief that God gave us free will which means we can chose truth or falsity. Our senses are fallible, but God must have given us some ability to trust our sense. Otherwise, we would not be able to choose correctly. Free will entails that we have the ability to choose correctly or incorrectly, so we must have the ability to choose either.
For Descartes, being human means being certain of one’s self: as an individual. Thought, reason and rationalism are the tools we use to apprehend knowledge and the world. Each person has a triadic existence: a mind in a body in the world. This was a significant idea because it stressed the certainty of individualism. It was also significant because Descartes was able to form a complete philosophical system based upon this one doctrine of the certainty of the self. This is not a “self-ish” perspective. It is just Descartes’ view that being human starts with the self. This would seem to indicate that Descartes meant that knowledge does not depend on the external world. Knowledge is a product of the rational mind. The mind and body interact but they are separate.
This is an ongoing philosophical debate. What knowledge comes completely from thought? What knowledge do we get from the world?