The essence may be roughly defined as that measure or standard which corresponds to the variation of immediate being, and yet remains identical in all variation. Or, if we like, we may say that this immediate being, which, as derivative, may now be called existence[1], has its ground in the essence. The essence is the ground of existence: and essence which exists is a 'thing,' Such an existing essence or thing subsists in its properties; and these properties are only found in the thing. Thus the essence, when it comes into existence as a thing, turns out to be a mere phenomenon or appearance.--Such briefly stated is the development of essence proper into appearance. With the idea of essential being--a permanent which yet changes, there emerge the problems connected with the double aspect of relation as identity and difference,--the favourite categories of reflection. These terms indeed the popular logician would fain avoid as savouring of pedantic accuracy, and prefers the psychological titles of similarity and contrast. These, he tends to insinuate, are unique experiences, direct feelings, beyond which it is impossible to go in analysis. The logician, on the other hand, must insist on dealing with the more radical phase of the terms. And he must note their essential interdependence and their intrinsic contradictoriness. Abstract sameness, or sameness which does not presuppose a tinge of difference, is a fiction of weak thought, which wishes to simplify the subtlety of nature. Identity is a relative term, and for that very reason presupposes difference: and for the same reason difference presupposes identity and is meaningless without it. The whole dispute about 'Personal Identity,' as it descends from one English psychologist to another, is enveloped in the obscurity which springs from failure to grasp the logical antinomy on which the question turns. When I feel that my friend whom I have not met for years is still the same, should I take the trouble to express myself in this manner, unless with reference to the difference betwixt Then and Now? If I remark that two men are different, would the remark be worth making or hearing unless there was some identity which made that difference all the more striking? The essence is, in short, the unity of sameness and difference: and when so apprehended, it is the ground by which we explain existence. The essence, ground, or possibility, is at once itself and not itself: if it is self-identical, it is for the same reason self-distinguishing. If it is to be itself, it can only be so by negativing what in it is other than itself. The affirmation of self implies the negation of the other of self,--the redintegration (though not the blank absorption) of the other in it. This is the _crux_ which lies in _Ex nihilo nihil fit:_ what exists must not be other than the essence (the effect not more than the cause), and yet unless it is other and different, there has been no passage from essence to existence.