Descartes does not often use the word “physics” (physica, physicos, or physique) in his writings. Perhaps his boldest and most famous pronouncement about physics comes in the preface to the 1647 French translation of his Principles of Philosophy (1644). There he explains what a man must do to apply himself to “True Philosophy.” He first must study metaphysics, and then, the
second [thing he must study] is Physics, in which after having discovered the true Principles of material things, one examines, in general, the composition of the whole universe, and then, in particular the nature of this earth and of all the bodies which are most commonly found around it, like air, water, fire, the lodestone, and other minerals.... Philosophy as a whole is like a tree; of which the roots are Metaphysics, the trunk is Physics and the branches emerging from the trunk are all the other branches of knowledge ... [namely,] Medicine, Mechanics and Ethics.(AT IXB 14, MM 24)
Parts II, III, and IV of the Principles are partly dedicated to “Physics,” yet well before 1644 Descartes was engaged with physical problems and seems to have begun his work in “physics” with his Dioptics, which was composed in the early 1630s. However, in his correspondence, the first work he sometimes refers to as “my Physics” is found in The World (see, e.g., AT I 348, CSMK 52). In both the physics of The World (1633) and of the later Principles, Descartes lays out a general account of matter and motion as well as a vortex theory of planetary motion. In what follows, the notable differences in how Descartes approaches some of the same issues in these two works are highlighted.
In The World, Descartes outlines his physics in a quite realistic manner. He lists three basic elements (fire, air, earth), which can be used to explain all natural phenomena by means of the “motion, size, shape and the arrangement of ... [their] parts” (AT XI 26, G 18). Descartes also writes: