20 PRINCIPLES OF SCIENCE. 



tween which no analogy whatever exists. It is 

 not legitimate to make use of analogy between? 

 flexible and elastic, any more than between 

 elastic and expansible bodies ; much less be- 

 tween matter which is ponderable and dark, 

 and such as is imponderable and luminous ; 

 between that which is passive and opake, and 

 that which is essentially active and transparent ; 

 between a cloud of dust, and the rays of light ; 

 between the natural obscurity of this globe of 

 earth, and the illumination and splendor of the 

 sun. 



In order, therefore, to prosecute to a suc- 

 cessful termination, any branch of science, it is 

 of the utmost importance, that we should be in 

 full possession of the first principles, on which 

 that science depends ; subsisting as causes 

 from whence the effects proceed ; and that the 

 definitions we employ, should be determinate 

 and precise, expressive of the thing signified. 

 This previous and antecedent knowledge, not 

 only comprehends a knowledge of the nature of 

 the subject itself, and of the terms by which its 

 existence is known and understood, but also 

 the various attributes belonging to it; the 

 nature of which will form the subject of the next 

 chapter. 



