SCIENTIFIC INVESTIGATION 89 



of nature that of gravitation. The method by which 

 men of science establish the existence of that law is ex- 

 actly 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, thor- 

 ough, 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 parlor 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 win- 

 dow is open, and you observe the mark of a dirty hand 

 on the window-frame, and perhaps, in addition to that, 

 you notice the impress of a hob-nailed shoe on the 

 gravel outside. All these phenomena have struck your 

 attention instantly, and before two seconds have passed 

 you say, " Oh, somebody has broken open the window, 

 entered the room, and run off with the spoons and the 

 tea-pot ! " That speech is out of your mouth in a mo- 

 ment. And you will probably add, " I know there has; 

 I am quite sure of it ! " You mean to say exactly what 

 you know; but in reality you are giving expression to 

 what is, in all essential particulars, an hypothesis. You 



