THE PRINCIPLES OF SCIENCE. 



yet we detect there the same elements of which our 

 own limbs are composed. The same natural laws can be 

 detected in operation in every part of the universe within 

 the scope of our instruments ; and doubtless these laws are 

 obeyed irrespective of distance, time and circumstance. 



It is the prerogative of Intellect to discover what is 

 uniform and unchanging in the phenomena around us. 

 So far as object is different from object, knowledge is 

 useless and inference impossible. But so far as object 

 resembles object, we can pass from one to the other. In 

 proportion as resemblance is deeper and more general, the 

 commanding powers of knowledge become more wonder- 

 ful. Identity in one or other of its phases is thus always 

 the bridge by which we pass in inference from case to 

 case ; and it is my purpose in this treatise to trace out the 

 various forms in which the one same process of reasoning 

 presents itself in the ever-growing achievements of 

 Scientific Method. 



The Powers of Mind concerned in the Creation of 



Science. 



It is no part of the purpose of this work to investigate 

 the nature of mind, except so far as its powers are 

 requisite to the formation of Science. In this place I 

 need only point out that the mental powers engaged in 

 knowledge are probably three in number. They are 

 substantially as Mr. Bain has stated them a : 



1 . The Power of Discrimination. 



2. The Power of .Detecting Identity. 



3. The Power of Retention. 



We exert the first power in every act of perception. 

 Hardly can we have a sensation or feeling unless we 

 discriminate it from something else which preceded. 



a 'The Senses and the Intellect,' Second Ed., pp. 5, 325, &c. 



