Up until now I have described the earth and indeed the whole visible universe as if it were a machine: I have considered only the various shapes and movements of its parts. But our senses show us much else besides, namely: colors, smells, sounds, and such-like.
With this statement Descartes begins the last part of his treatise, the treatment of our sensations. There is nothing resembling our sensations out in the world, on Descartes' picture. In the mechanistic, mathematical model he presents, there are only the properties that can be logically derived from extension (such as shape and motion). Colors, odors, tastes, hungers, etc. are not derivable from extension, and so they do not exist in the physical world. However, these properties are a large part of our experience of the world (in fact, they are almost our entire experience of the world), and so he deigns in the last section of the last chapter to deal with them. Sensations, he tells us, arise as a result of the interaction of our physical organs with particles of matter, followed by the interaction of our physical organs with our mind. His theory of sensation is presented in much greater detail in a later work entitled, On Man.