Dualism, simply put, is the belief that something is composed of two fundamentally different components, and it was around long before Descartes put pen to page. Cartesian Dualism deals specifically with the dual existence of man.
Can you explain Cartesian Dualism and how Descartes' philosophical endeavors led him to dualism?
Descartes believed that a man consisted of
Matter: The physical stuff that walks, talks, and plays the accordion.
Mind: The nonphysical substance (sometimes equated with the soul) that thinks, doubts, and remembers the tune to "Lady of Spain."
Descartes believed in a mechanistic view of the material world — that matter goes about its business and follows its own laws, except when it is interfered with by the mind. Man's mind, then, simply "pulls the levers" of the body to do its bidding. Exactly how the nonphysical mind interacts with the physical body is a point of contention. Descartes believed that the pineal gland in the brain was the locus of interaction between the mind and body because he believed that this gland was the only part of the brain that wasn't a duplicate.
It's important to remember that, for Descartes, the brain and the mind are not the same thing. The brain serves, in part, as a connection between the mind and the body, but because it is a physical, changeable thing, it is not the actual mind. Man's mind is whole and indivisible, whereas his body can be changed. You can cut your hair, remove your appendix, or even lose a limb, but that loss in no way reduces your mind.
Descartes also believed that man was the only dualistic creature. He placed animals in the realm of the purely physical, mechanistic world, acting purely on instinct and on the laws of nature.
Descartes was led to his dualistic theories in part from his most famous philosophical endeavor — to place into doubt all that could be doubted in the hope of arriving at a basic, undeniable truth. That resulted in his famous Cogito ergo sum — I think, therefore I am. Descartes could doubt the existence of the physical world and that even his own body actually existed, but he could not doubt the idea that his mind existed because doubting is a thought process. The very act of doubting one's existence proves that one actually exists; otherwise, who is doing the doubting?
Through his process of doubting, he recognized that, regardless of what the changeable physical world was really like, his mind was still whole and unchanged, and therefore somehow separate from that physical world.