The coherence and consistency of being was, it appeared in the last chapter, only to be maintained by assuming it to fall into two planes, or orders, always however relative to each other. The need of a measure forced itself upon even the superficial student. In the ordinary business of economical life one commodity of common use or of general acceptability steps into the place of a common measure. At first it is no more than one amongst many, a more suitable and convenient means of discharging the task of mensuration. But gradually it draws away into a world of its own, and acquires in common estimation a unique and peculiar dignity. It becomes a commodity of a higher order than the common, and is even treated as if it had intrinsic and inherent worth, apart from all relations of exchange. In a further stage it rises to rank as an invariable and almost supersensible standard, which amid all the fluctuations of currency tends to remain unchanged. One loses sight of the movement out of which it grew and in which it exists--the social give and take, the interaction of individual needs and general opinion. The characteristic feature of this sphere of thought is the perpetual antithesis of terms. And its tragedy is the result of the tendency to separate the terms, and treat them as independently real. It matters not how often this error may be detected. Each side of the antithesis no doubt reflects itself upon the other. But we as constantly fail to note that reflection. Even the philosopher who most loudly preaches relativity falls into the common trap, and speaks of relatives as ultimate and absolute. He talks of an Unknowable, as if it could be without a Known (or Knowable): whereas no such term fully manifests itself. Each term owes its distinct existence to its correlative: each gives itself over to the other, and invests it with meaning and authority. Accordingly when even the ordinary mind, which takes these categories as they are given, is asked what each means, it can only reply by referring to the other. A cause is _that_ which has an effect. The dialectic in the nature of thought,--its self-revising self-conscious nature--which was concealed in the First Part of Logic, where one term, when carried to its extreme, passed over into another, is made obvious in the Second Part, where each term postulates and even points to its correlative, and, however it may be contra-distinguished, cannot be thought without it. Thus, force is a meaningless abstraction without the correlative expression or utterance of force: and matter means nothing except in its distinction from, and yet reflection on, form. These, it may be said, are simple and tautological statements. They are principles, however, which every day sees disregarded. Have they, for example, been remembered by those theorists who tell us that everything is ultimately reducible to matter, or who propose to improve upon that theory by explaining that matter is after all only