  While working on the parhelia, Descartes conceived the idea for a very ambitious treatise. He wrote to Mersenne that he had decided not to explain “just one phenomenon” (the parhelia), but rather to compose a treatise in which he explained “all the phenomena of nature, that is to say, the whole of physics” (1:70). This work eventually became The World, which was to have had three parts: on light (a general treatise on visible, or material, nature), on man (a treatise of physiology), and on the soul. Only the first two survive (and perhaps only they were ever written), as the Treatise on Light and Treatise on Man. In these works, which Descartes decided to suppress upon learning of the condemnation of Galileo (1:270, 305), he offered a comprehensive vision of the universe as constituted from a bare form of matter having only length, breadth, and depth (three-dimensional volume) and carved up into particles with size and shape, which may be in motion or at rest, and which interact through laws of motion enforced by God (11:33–4). These works contained a description of the visible universe as a single physical system in which all its operations, from the formation of planets and the transmission of light from the sun, to the physiological processes of human and nonhuman animal bodies, can be explained through the mechanism of matter arranged into shapes and structures and moving according to three laws of motion. In fact, his explanations in the World and the subsequent Principles made little use of the three laws of motion in other than a  qualitative manner. The laws sustained the notion that matter moves regularly (in a straight line) and that upon impact bits of matter  alter their motions in regular ways—something that happens constantly in the full universe (the  “plenum”) conceived  by Descartes.