How does Descartes define what it means to be human, and what is the signifcance of the definition?

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?