§ 1. If we are, in the end, to attach any definite intelligible meaning to the distinction between things as they really are and things as they merely appear to be, we must clearly have some universal criterion or test by which the distinction may be made. This criterion must be, in the first place, infallible; that is, must be such that we cannot doubt its validity without falling into a contradiction in our thought; and, in the next, it must be a characteristic belonging to all reality, as such, and to nothing else. Thus our criterion must, in the technical language of Logic, be the predicate of an exclusive proposition of which reality is the subject; we must be able to say, “Only the real possesses the quality or mark _X_.” The argument of our last chapter should already have suggested that we have such a criterion in the principle that “what is real is not self-contradictory, and what is self-contradictory is not real.” Freedom from contradiction is a characteristic which belongs to everything that is real and ultimately to nothing else, and we may therefore use it as our test or criterion of reality. For, as we have seen in the last chapter, it is precisely our inability, without doing violence to the fundamental structure of our intellect, to accept the self-contradictory as real which first leads to the drawing of a distinction between the real and the merely apparent; on the other hand, where we find no contradiction in thought or experience, we have no valid ground for doubting that the contents of our experience and thinking are truly real. In every application, even the most simple and rudimentary, of the distinction between what really is and what only seems, we are proceeding upon the assumption that, if things as we find them are self-contradictory, we are not yet in possession of the truth about them; while, on the other hand, we may legitimately treat the results of our thinking and experience as fully true until they are shown to involve contradiction. Thus, in setting up the proposition “What is real is never self-contradictory” as a universal criterion, we are only putting into explicit form, and proposing to apply universally, a principle involved in all rational reflection on the course of things. Audacious as the attempt to make such a general statement about the whole universe of being appears, it is an audacity to which we are fully committed from the first moment of our refusing to accept both sides of a contradiction as true.