The difference between the conceptions of reality held by Aristotle and Plato respectively is that where Plato said Being, Essence or Substance (οὐσία), Aristotle said Activity (ἐνέργεια). To be is to act, to be active. To the outsider--the plain man of philosophic legend, it seems at first that a thing must _be_ before it can _do_: that you must have an agent before you get an action. And, in a way, Aristotle admits this not quite satisfactory criticism. Every activity presupposes, he allows, a power to act, a potentiality: every actual presupposes an implicit or a mere possibility. Existence seems, as it were, to be doubled; or the mere surface-being is turned into a subject which has a predicate. But if the existence is to be real, it has to include both elements, and with the latter or the actuality, as its crown. Nor is this all. The possibility which issues forth in action may be fairly called self-realisation. That is to say: A--the hypothetical agent--acts, does something: and in so doing, seems to go forth and beyond itself, to externalise itself. Or, A is acted upon, and thus seems to be diminished. But what it externalises, or puts forth, is after all what it _is_: it puts forth itself: and, on the other hand, if it be a patient, it is no less an agent and self-limitative. What a thing really is, is what it _makes_ itself be: what it allows itself to be made, that it really is. Yet further, if the word self-realisation be taken in its fullness of meaning,--if there be really a _self,_ and it be realised, then this self-realisation, which is the truth or more developed conception of being, seems to imply or postulate in it a self-consciousness, an awareness of the process of completed being,--completed in its return from utterance of possibility to self-fruition or in its re-assumption of itself.