THE CAUSES OF THE xi 



verification, and to take care, moreover, that this 

 is done intentionally, and not left to a mere acci- 

 dent, as in the case of the apples. And in science, 

 as in common life, our confidence in a law is in 

 exact proportion to the absence of variation in 

 the result of our experimental verifications. For 

 instance, if you let go your grasp of an article 

 you may have in your hand, it will immediately 

 fall to the ground. That is a very common veri- 

 fication of one of the best established laws of 

 nature that of gravitation. The method by 

 which men of science establish the existence of 

 that law is exactly the same as that by which we 

 have established the trivial proposition about the 

 sourness of hard and green apples. But we believe 

 it in such an extensive, thorough, and unhesitat- 

 ing manner because the universal experience of 

 mankind verifies it, and we can verify it ourselves 

 at any time ; and that is the strongest possible 

 foundation on which any natural law can rest. 



So much, then, by way of proof that the method 

 of establishing laws in science is exactly the same 

 as that pursued in common life. Let us now turn 

 to another matter (though really it is but another 

 phase of the same question), and that is, the 

 method by which, from the relations of certain 

 phenomena, we prove that some stand in the posi- 

 tion of causes towards the others. 



I want to put the case clearly before you, and I 

 will therefore show you what I mean by another 



