142 HISTORY OF PHYSICAL ASTRONOMY. 



a maxim, in the first sentence of his book, that a 

 person who seeks for truth must, once in his life, 

 doubt of all that he most believes. Conceiving 

 himself thus to have stripped himself of all his 

 belief on all subjects, in order to resume that part 

 of it which merits to be retained, he begins with 

 his celebrated assertion, " I think, therefore I am ;" 

 which appears to him a certain and immoveable 

 principle, by means of which he may proceed to 

 something more. Accordingly, to this he soon 

 adds the idea, and hence the certain existence, of 

 God and his perfections. He then asserts it to be 

 also manifest, that a vacuum in any part of the 

 universe is impossible; the whole must be filled 

 with matter, and the matter must be divided into 

 equal angular parts, this being the most simple, 

 and therefore the most natural supposition 3 . This 

 matter being in motion, the parts are necessarily 

 ground into a spherical form ; and the corners thus 

 rubbed off (like filings or sawdust) form a second 

 and more subtle matter 4 . There is, besides, a third 

 kind of matter, of parts more coarse and less fitted 

 for motion. The first matter makes luminous 

 bodies, as the sun, and the fixed stars ; the second 

 is the transparent substance of the skies ; the third 

 is the material of opaque bodies, as the earth, 

 planets, and comets. We may suppose, also 5 , that 

 the motions of these parts take the form of revolv- 

 ing circular currents 6 , or vortices. By this means, 

 3 Prin. p. 58. 4 lb. p. 59 8 IK p. 56. 6 Ib. p. 61. 



