44 THE PKINCIPLES OF SCIENCE. (CHAP. 



infinitely more common than that of likeness. One thing 

 may resemble a great many other things, but then it differs 

 from all remaining things in the world. Diversity may 

 almost be said to constitute life, being to thought what 

 motion is to a river. The perception of an object involves 

 its discrimination from all other objects. But we may 

 nevertheless be said to detect resemblance as often as we 

 detect difference. We cannot, in fact, assert the existence 

 of a difference, without at the same time implying the 

 existence of an agreement. 



If I compare mercury, for instance, with other metals, 

 and decide that it is not solid, here is a difference between 

 mercury and solid things, expressed in a negative propo- 

 sition ; but there must be implied, at the same time, an 

 agreement between mercury and the other substances 

 which are not solid. As it is impossible to separate the 

 vowels of the alphabet from the consonants without at the 

 same time separating the consonants from the vowels, so I 

 cannot select as the object of thought solid tilings, without 

 thereby throwing together into another class all things 

 which are not solid. The very fact of not possessing a 

 quality, constitutes a new quality which may be the ground 

 of judgment and classification. In this point of view, 

 agreement and difference are ever the two sides of the same 

 act of intellect, and it becomes equally possible to express 

 the same judgment in the one or other aspect. 



Between affirmation and negation there is accordingly a 

 perfect equilibrium. Every affirmative proposition implies 

 a negative one, and vice versd. It is even a matter of in- 

 difference, in a logical point of view, whether a positive or 

 negative term be used to denote a given quality and the 

 class of things possessing it. If the ordinary state of a 

 man's body be called good health, then in other circumstances 

 he is said not to be in good health ; but we might equally 

 describe him in the latter state as sickly, and in his normal 

 condition he would be not sickly. Animal and vegetable 

 substances are now called organic, so that the other sub- 

 stances, forming an immensely greater part of the globe, are 

 described negatively as inorganic. But we might, with at 

 least equal logical correctness, have described the prepon- 

 derating class of substances as mineral, and then vegetable 

 and animal substances would have been non-mineral. 



