MATTER THEORY AND MECHANICS

Part II of the Principia deals with the foundational principles of Descartes' physical theory, which take the form of a synthesis of matter theory and mechanics. Descartes pushed this enterprise further than any of his contemporaries, and his cosmology, which we shall be looking at in the next chapter, is the first to present a full model of the universe that integrates mechanical and matter–theoretic considerations. Such an integration was extremely problematic, and there are questions about whether it is even possible. Mechanics deals with physical processes in terms of the motions undergone by bodies and the nature of the forces responsible for these motions. Matter theory deals with how the physical behaviour of a body is determined by what it is made of, and in the seventeenth century it typically achieves this in a corpuscularian fashion, by investigating how the nature and arrangement of the constituent parts of a body determine its behaviour. Mechanical and matter–theoretic approaches to physical theory are very different; they engage fundamentally different kinds of considerations, and on the face of it offer explanations of different phenomena. We do not explain how levers, inclined planes, screws, and pulleys work in terms of matter theory. Correlatively, it is far from clear that the appropriate form of explanation of the phenomena of burning, fermentation, and differences between fluids and solids is in terms of mechanics.