Descartes saw the mind and body as two distinct things, each capable of existing on its own. This view is called dualism, and it is the source of the phrase “the ghost in the machine”: the body is the machine, and the mind is the ghost. Trouble is, how do the ghost and the machine interact?
In this article in the New Statesman, Colin McGinn talks about “the” five positions on the philosophy of mind, including Cartesian dualism, and does a pretty good job of describing them. But all five positions, as he notes, are flawed; and he begins to wonder if he should call himself a “mysterian”, i.e., one who considers mind simply a mystery.
But I’ll note that there’s a sixth position that he’s ignored: that of Aristotle and St. Thomas, which Ed Feser among others refers to as hylemorphic dualism.
Not a bad article, though. Worth your time.