tacr. in.] DISSERTATION SECOND. Ill 



that method of demonstrating its principles. His argument 

 was, that motion is a state of body, and that body or matter 

 cannot change its own state. This was his demonstration of 

 the first proposition, from which the second followed ne- 

 cessarily. 



The evidence produced for the third, or the preserva- 

 tion of the same quantity of motion in the universe, is 

 founded on the immutability of the Divine nature, and is 

 an instance of the intolerable presumption which so often 

 distinguished the reasonings of this philosopher. Though 

 the immutability of the Divine nature will readily be ad- 

 mitted, it remains to be shown, that the continuance of the 

 same quantity of motion in the universe is a consequence 

 of it. This, indeed, cannot be shown, for that quantity, 

 in the sense in which Descartes understood it, is so far 

 from being preserved uniform, that it varies continually 

 from one instant to another. It is nevertheless true, that 

 the quantity of motion in the universe, when rightly esti- 

 mated, is invariable, that is, when reduced to the direc- 

 tion of three axes at right angles to one another, and 

 when opposite motions are supposed to have opposite 

 signs. This is a truth now perfectly understood, and is a 

 corollary to the equality of action and reaction, in conse- 

 quence of which, whatever motion is communicated in one 

 direction, is either lost in that direction, or generated in the 

 opposite. This, however, is quite different from the pro- 

 position of Descartes, and if expressed in his language, 

 would assert, not that the sura, but that the difference of 

 the opposite motions in the universe remains constantly 

 the same. When he proceeds, by help of the principle 

 which he had thus mistaken, to determine the laws of the 

 collision of bodies, his conclusions are almost all false, and 

 have, indeed, such a want of consistency and analogy with 

 one another, as ought, in the eyes of a mathematician, to 





