OF NATURAL PHILOSOPHY. 101 



mena of double refraction, and the exhibition of 

 periodical colours ; which in the actual case is one 

 of the most important, viz. the relation of constant 

 association, inasmuch as it asserts that in whatever 

 individual the one character is found, the other will 

 invariably be found also. 



(92.) These two lights, in which the announce- 

 ment of a general law may be regarded, though at 

 bottom they come to the same thing, yet differ 

 widely in their influence on our minds. The for- 

 mer exhibits a law as little more than a kind of arti- 

 ficial memory ; but in the latter it becomes a step 

 in philosophical investigation, leading directly to the 

 consideration of a proximate, if not an ultimate, 

 cause ; inasmuch as, whenever two phenomena are 

 observed to be invariably connected together, we 

 conclude them to be related to each other, either 

 as cause and effect, or as common effects of a single 

 cause. 



(93.) There is still another light in which we 

 may regard a law of the kind in question, viz. as a 

 proposition asserting the mutual connection, or in 

 some cases the entire identity, of two classes of in- 

 dividuals (whether individual objects or individual 

 facts) ; and this is, perhaps, the simplest and most 

 instructive way in which it can be conceived, 

 and that which furnishes the readiest handle to 

 further generalization in the raising of yet higher 

 axioms. For example : in the case above men- 

 tioned, if observation had enabled us to establish 

 the existence of a class of bodies possessing the 

 property of double refraction, and observations 

 of another kind had, independently of the former, led 

 H 3 



