THE PRINCIPLES OF SCIENCE. 



Laws of Identity or Difference. Are they Laws of Thought 

 or Laws of Things 1 Do they belong to mind or to 

 material nature I On the one hand it may be said 

 that science is a purely mental existence, and must 

 therefore conform to the laws of that which formed it. 

 Science is in the mind and not in the things, and the 

 properties of mind are therefore all important. It is true 

 that these laws are verified in the observation of the 

 exterior world ; and it would seem that they might have 

 been gathered and proved by generalisation, had they 

 not already been in our possession. But on the other 

 hand, it may well be urged that we cannot prove these 

 laws by any process of reasoning or observation, be- 

 cause the laws themselves are presupposed, as Leibnitz 

 acutely remarked, in the very notion of a proof. They 

 are the prior conditions of all thought and all know- 

 ledge, and even to question their truth is to allow 

 them true. Hartley ingeniously refined upon this argu- 

 ment, remarking that if the fundamental laws of logic 

 be not certain, there must exist a logic of a second 

 order whereby we may determine the degree of uncer- 

 tainty : if the second logic be not certain, there must 

 be a third, and so on ad inftnitum. Thus we must sup- 

 pose either that absolutely certain laws of thought exist, 

 or that there is no such thing as certainty whatever e . 



Logicians, indeed, appear to me to have paid insuf- 

 ficient attention to the fact that mistakes in reasoning 

 are always likely to occur. The Laws of Thought are 

 often called necessary laws, that is, laws which cannot 

 but be obeyed. Yet as a matter of fact who is there 

 that does not often fail to obey them ? They are the 

 laws which the mind ought to obey rather than what 

 it always does obey. Our thoughts cannot be the 

 criterion of truth, for we often have to acknowledge 



e Hartley on Man, vol. i. p. 359. 



