METHOD OF DISCOVERY. 61 



tion in the result of our experimental verifica- 

 tions. For instance, if you let go your grasp of an 

 article you may have in your hand, it will imme- 

 diately fall to the ground. That is a very common 

 verification 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 estab- 

 lished the trivial proposition about the sourness of 

 hard and green apples. But we believe it in such an 

 extensive, thorough, and unhesitating 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 position 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 familiar 

 example. I will suppose that one of you, on coming 

 down in the morning to the parlour of your house, 

 finds that a tea-pot and some spoons which had been 

 left in the room on the previous evening are gone, 

 — the window is open, and you observe the mark of a 

 dirty hand on the window-frame, and perhaps, in addi- 



