4 The Theory of the Aether 



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, correspond- 

 ing 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 endeavour 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 



