The term _ousia_ in most of its senses had come to be convertible with two other terms, _hypostasis_ (ὑπόστασις) and _hyparxis_ (ὕπαρξις). The latter of these played but a small part in Christian theology, and may be disregarded here.[572] The term _hypostasis_ is the conjugate of the verb ὑφιστάναι, which had come into use as a more emphatic form than εἶναι. It followed almost all the senses of _ousia_. Thus it was contrasted with phenomenal existence not merely in the Platonic but in the conventional sense; e.g. of things that take place in the sky, some are appearances, some have a substantial existence, καθ’ ὑπόστασιν.[573] It also, like _ousia_, is used of that which has an actual as compared with a potential existence;[574] also of that which has an objective existence in the world, and not merely exists in the thinking subject.[575] Hence when things came into being, οὐσία was said ὑφιστάναι.[576] Moreover, in one of its chief uses, namely that in which it designated the permanent element in objects of thought, the term ὀυσία had sometimes been replaced by the term ὑπόστασις.[577] When, therefore, the use of _ousia_ in its Neo-Platonic sense prevailed, there arose a tendency to differentiate the two terms, and to designate that which in Aristotle had been πρώτη οὐσία by the term ὑπόστασις. This is expressed by Athanasius when he says: “Ousia signifies community,” while “hypostasis has property which is not common to the hypostases of the same ousia;”[578] and even more clearly by Basil.[579] There was the more reason for the growth of the distinction, because the term _homoousios_ lent itself more readily to a Sabellian Christology. This was anticipated by Irenæus in his polemic against the Valentinian heresy of the emission of Æons. _Ousiai_, in the sense of genera and species, might be merely conceptions in the mind: the alternative was that of their having an existence of their own.[580] So that _hypostasis_ came in certain schools of thought to be the term for the _substantia concreta_, the individual, the οὐσία ἄτομος of Galen.[581] The distinction, however, was far from being universally recognized. The clearest and most elaborate exposition of it is contained in a letter of Basil to his brother Gregory, who was evidently not quite clear upon the point.[582] The result was, that just as ὑπόστασις had been used to express one of the senses of οὐσία, so a new term came into use to define more precisely the sense of ὑπόστασις. Its origin is probably to be traced to the interchange of documents between East and West, which leading to a difficulty in regard to this use of ὑπόστασις, ended in the introduction of a third term.