The Mind: Cartesian Substance Dualism: Part 1

Theories about the nature of human mind reach back at least as far as Ancient Greece. Plato wrote about it and so did Aristotle. However, Philosophy of Mind in the modern era begins with René Descartes. Descartes was a French philosopher, mathematician, scientist and writer during the 17th century.
He is also responsible for a skeptical method, sometimes referred to as Cartesian Doubt, where nothing is taken for granted as a given. It was through the employment of this method, in an attempt to prove the immortality of the soul, that we find the meat of his argument for substance dualism and his theory of mind. This exercise in doubt is chronicled in his [I]Meditations on First Philosophy[/I].
Without descending into a full-blown explanation of the [I]Meditations[/I], much of which is off topic, substance dualism can be explained more or less as follows. There are, effectively, two substances which compose a person: material and immaterial substances. Material substances (bodies) are defined by being extensible (having shape and form in 3-dimensional space), having duration through time, and being subject to decay and death. Immaterial substances (minds) are defined primarily by not being extensible, not being subject to decay and death, and also having duration through time.
Descartes came to his conclusion about these immaterial substances by his skeptical method. He posed himself the question, is there anything about which i cannot doubt? His answer to this question was, in the [I]Meditations[/I], "I am a thinking thing." The more famous version, The Cogito ("cogito, ergo, sum") is often mis-attributed to the [I]Mediations[/I]. It actually comes from his [I]Discourse on Method [/I]and translates to "I think, therefore, I am." In either case, the gist is the same, the immaterial is for Descartes the principle substance.
His argument for the statement that, "I am a thinking thing," essentially goes like this:
While I may be able to doubt the existence of my body, or the entire material world, I cannot doubt the existence of my own mind. When I try to doubt it, I am confronted with the fact that someone must be doubting. That I can doubt my existence proves my existence.
These claims came under fire almost as soon as the work was published and have been receiving criticism since. In Part 2 of Cartesian Substance Dualism, some of the objections that have been raised will be examined.,