He must take for granted that his proposition, when he proposes it, is and must be what it is. This is all we want. The law of contradiction thus vindicates itself. It cannot be denied without being assented to, for the person who denies it must assume that he is denying it; in other words, he must assume that he is saying what he is saying, and he must admit that the contrary supposition—to wit, that he is saying what he is not saying—involves a contradiction. Thus the law is established. It proves the existence of, at any rate, one necessary truth or law of reason; and if there can be one, why can there not be many? Indeed, the law of contradiction is not so much one special necessary truth, as the generalisation or general form, and exponent of all ideas (and their name is legion) whose opposites involve a mental contradiction. The reader need scarcely be informed that the law of contradiction has no worth or merit of its own. Looked at in itself, it is trivial beyond triviality. It is merely convenient, as an abbreviated expression for the criterion of all necessary truth, the test being—do their opposites involve a mental contradiction?—are their opposites at variance with the law which declares that A is A? If they are—if they are equivalent to a denial that a thing is what it is—then the truths in question are necessary; if they do not involve this contradiction, the truths in question are contingent.
