THE PRINCIPLES OF SCIENCE. 



Laws of Identity and Difference. 



At the basis of all thought and science must lie the 

 laws which express the very nature and conditions of the 

 discriminating and identifying powers of mind. These 

 are the so-called Fundamental Laws of Thought, usually 

 stated as follows : 



1 . The Law of Identity. Whatever is, is. 



2. The Law of Contradiction. A thing cannot both be 



and not be. 



3. The Law of Duality. A thing must either be or 



not be. 



The first of these statements may perhaps be regarded 

 as a description of identity itself, if so fundamental a 

 notion can admit of description. A thing at any moment 

 is perfectly identical with itself, and if any person were 

 unaware of the meaning of the word identity we could 

 not better describe it than by such an example. 



The second law points out that contradictory attri 

 butes can never be joined together. The same object may 

 vary in its different parts ; here it may be black, and 

 there white ; at one time it may be hard and at another 

 time soft : but at the same time and place an attribute 

 cannot be both present and absent. Aristotle truly 

 described this law as the first of all axioms one of 

 which we need not seek for any demonstration. All 

 truths cannot be proved, otherwise there would be an 

 endless chain of demonstration ; and it is in self-evident 

 truths like this that we find the fittest foundation. 



The third of these laws completes the other two. It 

 asserts that at every step there are two possible alter 

 natives presence or absence, affirmation or negation. 

 Hence I propose to name this law the Law of Duality, 



c Metaphysics, Bk. III. chap. iii. 9-12, 



