Of these two meanings of ousia, namely “species” and “genus,” the former expressing the whole essence of a class-name or concept, the latter part of the essence, the former tended to prevail in earlier, the latter in later Greek philosophy. In the one, the knowledge of the ousia was completely unfolded in the definition, so that a definition was itself defined as “a proposition which expresses the ousia:”[561] in the latter, it was only in part so unfolded, so that it is necessary for us to know not only the ousia of objects of thought, for example, whether they fall within or without the class “body,” but also the species (εἴδη).[562]
But in the one meaning as in the other, the members of the same class, or the sub-classes of the same wider class, were spoken of as homoousioi: for example, there was an argument that animals should not be killed for food, on the ground that they belong to the same class as men, their souls being homoousioi with our own:[563] so men are homoousioi with one another, and Abraham washed the feet of the three strangers who came to him, thinking them to be men “of like substance” with himself.[564]
The difficulty of the whole conception in its application to God was felt and expressed. Some philosophers, as we have already seen, denied that such an application was possible. The tide of which Neo-Platonism was the most prominent wave placed God beyond ousia. Origen meets Celsus’s statement of that view by a recognition of the uncertainty which flowed from the uncertain meaning of the term.[565] The Christological controversies of the fourth century were complicated to no small extent from the existence of a neutral and conservative party, who met the dogmatists on both sides with the assertion that neither ousia nor hypostasis was predicable of God.[566] And, in spite of the acceptance of the Nicene formula, the great Christian mystic who most fully represents Neo-Platonism within the Christian Church, ventured more than a century later on to recur to the position that God has no ousia, but is hyperousios.[567] Even those who maintained the applicability of the term to God, denied the possibility of defining it when so applied to Him. In this they followed Philo: “Those who do not know the ousia of their own soul, how shall they give an accurate account of the soul of the universe?”[568] But in spite of these difficulties, the conservative feeling against the introduction of metaphysical terms into theology, and the philosophical doctrine of absolute transcendence, were overborne by the practical necessity of declaring that He is, and by the corollary that since He is, there must be an ousia of Him.
But when the conception of the one God as transcending numerical unity became dominant in the Christian Church, the term homoousios (ὁμοούσιος) was not unnaturally adopted to express the relation of God the Father to God the Son. It accentuated the doctrine that the Son was not a creature (κτίσμα); and so of the term as applied to the Holy Spirit. Those who maintained that the Holy Spirit was a creature, thereby maintained that He was severed from the essence of the Father.[569] The term occurs first in the sphere of Gnosticism, and expresses part of one of the two great conceptions as to the origin of the world.[570] It was rejected in its application to the world, but accepted within the sphere of Deity as an account of the origin of His plurality. But homoousios, though true, was insufficient. It expressed the unity, but did not give sufficient definition to the conception of the plurality. It was capable of being used by those who held the plurality to be merely modal or phenomenal.[571] It thus led to the use of another term, of which it is necessary to trace the history.
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.
So long as οὐσία and ὑπόστασις had been convertible terms, the one Latin word substantia, the etymological equivalent of ὑπόστασις, had sufficed for both. When the two words became differentiated in Greek, it became advisable to mark the difference. However, the word essentia, the natural equivalent for οὐσία, jarred upon a Latin ear.[583] Consequently substantia was claimed for οὐσία, while for ὑπόστασις a fresh equivalent had to be sought. This was found in persona, whose antecedents may be those of “a character in a play,” or of “person” in the juristic sense, a possible party to a contract, in which case Tertullian may have originated this usage.[584] Such Western practice would tend to stimulate the employment of the corresponding Greek term πρόσωπον, whose use hitherto seems to have been subordinate to that of ὑπόστασις.[585] And, finally, the philosophic terms φύσις and natura came into use. In the second century φύσις had been distinct from οὐσία and identical with Reason.[586] But in the fourth century it came to be identified with οὐσία,[587] and afterwards again distinguished from it, whereas the Monophysites identified it with ὑπόστασις.
To sum up, then. We have in Greek four terms, οὐσία, ὑπόστασις, πρόσωπον, φύσις, and in Latin three, substantia, persona, natura, the two series not being actually parallel even to the extent to which they are so in appearance. Times have changed since Tertullian’s[588] loose and vague usage caused no remark; when Jerome, thinking as a Latin, hesitates to speak of τρεῖς ὑποστάσεις, by which he understood tres substantias, and complains that he is looked upon as a heretic in the East in consequence. There is a remarkable saying of Athanasius which is capable of a wider application than he gave it: it runs as follows:[589] “They seemed to be ignorant of the fact that when we deal with words that require some training to understand them, different people may take them in senses not only differing but absolutely opposed to each other.”[590] Thus there was an indisposition to accept οὐσία. The phrase was not understanded of the people.[591] A reaction took place against the multiplicity of terms; but the simple and unstudied language of the childhood of Christianity, with its awe-struck sense of the ineffable nature of God, was but a fading memory, and on the other hand the tendency to trust in and insist upon the results of speculation was strong. Once indeed the Catholic doctrine was formulated, then, though not till then, the majority began to deprecate investigations as to the nature of God.