Book I Chapter 2 - § 2

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§ 2. Our declaration that the principle of the self-consistency of the real affords a certain and infallible criterion of reality, may probably provoke a sceptical doubt which is of such importance that we must give it full consideration before making any further advance. I state the difficulty in what appears to me its most reasonable and telling form. "Your alleged criterion," it will be said, "is simply the logical Law of Contradiction expressed in a novel and misleading way. Now, the Law of Contradiction, like all purely logical laws, is concerned not with real things, but exclusively with the concepts by which we think of them. When the logician lays it down as a fundamental truth of his science that A cannot be both B and not B, his A and B stand not for things "in the real world to which our thoughts have reference," but for concepts which we frame about the things. His law is thus purely what he calls it, a Law of Thought; he says, and says truly, "you cannot, at the same time, and in the same sense, think both that A is B, and that it is not B"; as to whether such a state of things, though unthinkable to us, may be real "as a fact," he makes no assertion. You take this law of our thinking, silently assume that it is also a law of the things about which we think, and go on to set it up as an infallible criterion of their reality. Your procedure is thus illegitimate, and your pretended criterion a thing of nought."[1]
 
Our reply to this common sceptical objection will incidentally throw an interesting light on what was said in the last chapter of the close connection between the problems of Logic and those of Metaphysics. In the first place, we may at least meet the sceptic with an effective tu quoque. It is you yourself, we may say, who are most open to the charge of illegitimate assumption. Your whole contention rests upon the assumption, for which you offer no justification, that because the Law of Contradiction is admittedly a law of thought, it is therefore only a law of thought; if you wish us to accept such a momentous conclusion, you ought at least to offer us something in the nature of a reason for it. Nor shall we stop here; we shall go on to argue that the sceptic's interpretation of the Law of Contradiction rests on a positive confusion. By a Law of Thought may be meant either (a) a psychological law, a true general statement as to the way in which we actually do think, or (b) a logical law, a true general statement as to the conditions under which our thinking is valid; the plausibility of the sceptical argument arises from an unconscious confusion between these two very different senses of the term. Now, in the first place, it seems doubtful whether the principle of contradiction is even true, if it is put forward as a psychological law. It would be, at least, very hard to say whether a human being is capable or not of holding at once and with equal conviction the truth of two contradictory propositions. Certainly it is not uncommon to meet persons who do fervently profess equal belief in propositions which we can see to be inconsistent; on the other hand, they are usually themselves unaware of the inconsistency. Whether, in all cases, they would, if made aware of the inconsistency, revise their belief, is a question which it is easier to ask than to answer. But it is at any rate certain that the logician does not intend his Law of Contradiction to be taken as a psychological proposition as to what I can or cannot succeed in believing. He means it to be understood in a purely logical sense, as a statement about the conditions under which any thought is valid. What he says is not that I cannot at once think that A is B and that it is not B, but that, if I think so, my thinking cannot be true. Now, to think truly about things is to think in accord with their real nature, to think of them as they really are, not as they merely appear to an imperfect apprehension to be; hence to say that non-contradiction is a fundamental condition of true thinking is as much as to say that it is a fundamental characteristic of real existence. Just because the Law of Contradiction is a logical law, it cannot be only a logical law, but must be a metaphysical law as well. If the sceptic is to retain his sceptical position, he must include Logic along with Metaphysics in the compass of his doubts, as the thorough-going sceptics of antiquity had the courage to do.

 [1] We shall meet this same difficulty again later on as the principle of the famous Kantian objection to the "ontological proof" of God's existence. Infra, Bk. IV. chap. 5, § 8.