44 THE PRINCIPLES OF SCIENCE. [chap. 



infinitely more common than tliat of likeness. One thing 

 nio.y 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 tliere nmst be implied, at the same time, an 

 agi'eement between mercury and the other substances 

 which are not solid. As it is impossil)le to separate the 

 voAvels of the alphaoet from the consonants without at the 

 same time separating the consonants from the vowels, so I 

 cannot select as the object of thought solid things, 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, 

 agieenient 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 aftirmation and negation there is accordingly a 

 perfect equilibrium. Every affirmative proposition implies 

 a negative one, and vice vcrsd. 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 he in good health ; but we might equally 

 describe him in tlie 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 suli- 

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

 described negatively as inorgardc But we nnght, with at 

 least equal logical correctness, have described the prepon- 

 derating class of sul)Stances as nnneral, and then vegetable 

 and animal substaiic(\s would have been non-mineral 



