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René Descartes intended the Principles of Philosophy to be his magnum opus—the synthesis of all his theories in physics and philosophy. The work, therefore, is full of information, but it is conveniently divided up into four easily digestible parts. Each of the parts is constructed as a collection of logically connected principles, conveniently numbered and titled.
Part I is the only part of the book that we, today, would call "philosophy." It is an account of Descartes's epistemology and his metaphysics. The rest of the book, beginning with Part II, deals with Descartes' natural philosophy, or what we would call "science." Descartes lays out the principles of his physics in Part II. In Part III he uses these principles to develop his theory of the structure of the universe and the solar system. Part IV uses these same principles to investigate the origins of the earth as well as a wide variety of earthly phenomena. The book concludes with bits and pieces of a theory of physiology and psychology, systems he would not work out in full until the publication of The Passions of the Soul in 1649.
The first strictly philosophical part of the Principles of Philosophy is largely a restatement of the conclusions Descartes drew in his earlier philosophical work, Meditations on First Philosophy. As in that work, he begins his discussion by calling all of our knowledge into doubt. His purpose here is not to argue that we know nothing, but to determine whether there is anything we can know for certain. By the seventh principle, he has identified one certain piece of knowledge among all our doubtful be liefs, a truth that cannot possibly be called into doubt: the fact that he exists. Using only this fact, a few principles of logic, and some allegedly innate ideas, he is able to prove the existence of God, the reliability of his faculty of reason whe n used correctly, the existence of an external world, and the nature of mind and of body.
Two conclusions from the first part stand out as particularly crucial to the project that the rest of the text will take on. First, Descartes ensures the trustworthiness of the methodology that will guide the rest of the discussion by proving that we can tr ust our clear and distinct perceptions to tell us the truth about the world. By establishing this claim, Descartes has given us a guarantee that so long as we continue to use his method correctly (by proceeding from the simplest self-evident principles to larger claims by way of indubitable chains of logic) we cannot fail to hit on actual fact. He secures this guarantee by establishing that God is responsible for the workings of our faculty of reason and that God, who is perfect in every way, would not de liberately mislead us by giving us a faulty faculty.
The second crucial conclusion of Part I is the claim that body is nothing but extended substance. The claim that body is nothing but extended substance allows Descartes to conflate physics into geometry and explain every phenomenon in the physical world with a few simple geometrically-based principles.
This attempt is the subject matter of Part II. It begins with a restatement of the argument for the claim that body is nothing but extended substance and goes on to explain away our intuitions that this is not the case. The rest of Descartes's physics is then deduced from the geometric properties of extended body. Central to the formulation of Descartes' physics are his discussion of space and of motion. Space, according to Descartes, is nothing but insensible body. In other words, space and body are really the same thing. To say that space is empty is incoherent; it is the same as saying that a water jug is empty when it contains nothing but air. Instead, he claims, space is a plenum, filled with indefinitely divisible body, or extended substance.
The individuation of particular bodies (such as planets, people, flowers, microscopic particles) from this continuous extended substance depends entirely on motion. Motion, in this view, takes on some strange properties. First of all, like shape, it is simply a way of being extended. In addition, in order to enable motion within the plenum, Descartes has to tell a complicated story about complete circles of motion effected throughout large segments of the plenum simultaneously. The mechanics of motion leads Descartes to the conclusion that there is an indefinite number of microscopic particles in the universe. Part II ends with Descartes' three laws of nature (all about motion).
Part III turns to the observable phenomena of the universe. Using only the principles established in Part II, Descartes is able to deduce the motions of the planets, the composition of all elements in the universe, and the properties of light, among other things. Probably influenced in large part by Galileo's recent run-in with the Catholic Church, Descartes gives an oddly complex account of the earth's motion, according to which the earth both moves and does not move. According to this picture the entire heavens act as a fluid vortex moving around the sun. Within the vortex, however, the earth does not change its position.
Finally, in Part IV, Descartes turns his explanatory principles loose on earth. He first gives an account of the earth's origin and then moves on to give explanations of gravity, magnetism, tides, heat, and the conclusions of chemistry. He ends the book with a discussion of human sensations and emotions.