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. § 3. But now suppose the sceptic takes this line. All our truth, he may say, is only relatively truth, and even the fundamental conditions of true thought are only valid _relatively_ and for us. What right have you to assume their _absolute_ validity, and to argue from it to the real constitution of things? Now, what does such a doubt mean, and is it rational? The answer to this question follows easily from what we have already learnt about the logical character of denial. Doubt, which is tentative denial, like negation, which is completed denial, logically presupposes positive knowledge of some kind or other. It is never rational to doubt the truth of a specific proposition except on the strength of your possession of positive truth with which the suggested judgment appears to be in conflict. This is, of course, obvious in cases where we hesitate to accept a statement as true on the ground that we do not see how to reconcile it with another specific statement already known, or believed, to be true. It is less obvious, but equally clear on reflection, in the cases where we suspend our judgment on the plea of insufficient evidence. Apart from positive knowledge, however defective, as to the kind and amount of evidence which _would_, if forthcoming, be sufficient to prove the proposition, expressions of doubt and of belief are equally impertinent; unless I know, to some extent at least, what evidence is wanted, how indeed am I to judge whether the evidence