There is a common impression that everything we believe ought to be capable of proof, or at least of being shown to be highly probable. Many people think that a belief for which no reason can be given is an unreasonable belief. In the main, this view is just. Almost all our common beliefs are inferred, or capable of being inferred, from other beliefs which give the reason for them even if the reason has been forgotten, or has never been consciously present to our minds.

But let’s imagine some insistent Socrates, who, whatever reason we give him, continues to demand a reason for the reason. Sooner or later we are driven to a point where we cannot find any further reason, and where it becomes almost certain that no further reason is even theoretically discoverable. Starting with the common beliefs of daily life, we can be driven back until we come to some general principle, or some instance of a general principle, which seems luminously evident, and is not capable of being deduced from anything more evident.

For example, the law of contradiction (a principle of logic) states that nothing can both have a certain property and not have it. This is self-evident, but it becomes even clearer when we consider an example: a particular rose which we see cannot be both red and not red. (It is of course possible that parts of the rose may be red and parts not red, or that the rose may be of a shade of pink which we hardly know whether to call red or not; but in the former case it is plain that the whole rose is not red, while in the latter case the answer becomes definite once we have decided on a precise definition of ‘red’.) The truth of such logical principles is self-evident to us.

In addition to general principles, the other kind of self-evident truths are those immediately derived from sensation. We call such truths ‘truths of perception’, and the judgments expressing them we will call ‘judgments of perception’. If I see a red patch of color, the fact that I am (at this moment) perceiving that patch of color is self-evident to me. It may be possible that I am mistaken about what that patch of color represents, whether an apple, or a fake wax apple. But it is impossible for me to be mistaken about the fact that I am experiencing a perception of a certain quality.

Another class of intuitive judgments, analogous to those of sense and yet distinct from them, are judgments of memory. One important point about self-evidence is made clear by the case of memory: that self-evidence has degrees. It is not a quality which is simply present or absent, but a quality which may be more or less present, in gradations ranging from absolute certainty down to an almost imperceptible faintness. Memories have a diminishing self-evidence as they become remoter and fainter.

Degrees of self-evidence are important in the theory of knowledge, since, if propositions may (as seems likely) have some degree of self-evidence without being true, we need not abandon all connection between self-evidence and truth, but merely say that, where there is a conflict, the more self-evident proposition should be retained and the less self-evident rejected. It seems, however, highly probable that two different notions are combined in ‘self-evidence’; that one of them, which corresponds to the highest degree of self-evidence, is really an infallible guarantee of truth, while the other, which corresponds to all the other degrees, does not give an infallible guarantee, only a greater or less presumption.

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