light and heat and influencing their motions, it followed from Descartes' principle that interplanetary space must be a plenum, occupied by matter imperceptible to the touch but capable of serving as the vehicle of force and light. This conclusion in turn determined the view which he adopted on the all-important question of the nature of matter.
Matter, in the Cartesian philosophy, is characterized not by impenetrability, or by any quality recognizable by the senses, but simply by extension; extension constitutes matter, and matter constitutes space. The basis of all things is a primitive, elementary, unique type of matter, boundless in extent and infinitely divisible. In the process of evolution of the universe three distinct forms of this matter have originated, corresponding respectively to the luminous matter of the sun, the transparent matter of interplanetary space, and the dense, opaque matter of the earth, "The first is constituted by what has been scraped off the other particles of matter when they were rounded; it moves with so much velocity that when it meets other bodies the force of its agitation causes it to be broken and divided by them into a heap of small particles that are of such a figure as to fill exactly all the holes and small interstices which they find around these bodies. The next type includes most of the rest of matter; its particles are spherical, and are very small compared with the bodies we see on the earth; but nevertheless they have a finite magnitude, so that they can be divided into others yet smaller. There exists in addition a third type exemplified by some kinds of matter-namely, those which, on account of their size and figure, cannot be so easily moved as the preceding. I will endeavor to show that all the bodies of the visible world are composed of these three forms of matter, as of three distinct elements; in fact, that the sun and the fixed stars are formed of the first of these elements, the interplanetary spaces of the second, and the earth, with the planets and comets, of the third. For, seeing that the sun and the fixed stars emit light, the heavens transmit it, and the earth, the planets, and the comets reflect it, it appears to me that there
