IPOSTASI. — This is the transcription of the Greek word ὑπόστασις, which has the same etymology as the Latin words substantia, subsistentia, suppositum and literally means that which stands beneath, that which remains beneath the flowing of appearances, that which thought places beneath phenomena. It is a highly important theological term, whose meaning was clarified as a result of vigorous controversies in the language of the Church, of the Fathers, and of Scholastic philosophy.
Apart from the common meaning it had in popular and literary usage, i., in Greek philosophy and among the Fathers, first signified the proper essence of a thing, as a synonym for οὐσία: what a thing really is, its fundamental and specific content: its proper nature (οὐσία). I. was therefore also a synonym for nature, which is conceived as the foundation of all the properties of a thing and as the principle of its specific activities. In s. Paul it precisely means foundation («i. of things hoped for», Hebr. 11, 1), principle («image of his principle», ibid. 1, 3), and also essence.
Secondly, in Greek philosophy and among the Fathers, i. acquired the meaning of «one who possesses subsistence, or essence or nature», thus becoming a synonym for an existing individual or person.
In having to express the notions of the Christian mysteries in terms of ordinary language, the Fathers found themselves confronted with possible misunderstandings and were compelled to give the terms a more proper and precise meaning. Thus, in the mystery of the Most Holy Trinity, the word i. became a source of ambiguity, both when it was said that there are three i. in the divinity, since this could be misunderstood as meaning three essences or substances or natures (the Arian heresy), and when it was said that there is one i. in God, since this could be misunderstood as meaning a subsistent nature as a person (the heresy of Sabellius). Hence s. Jerome had occasion to say that poison was concealed in the word i. (Epist. XV, to Pope Damasus). The Synod of Alexandria, in 362, left every believer free to profess either that there is one i. in God or that there are three i., precisely because of the twofold meaning of that word (essence-substance and person). It was not possible, however, to remain in ambiguity, and it was the purifying work of the Fathers, against the ambiguities of the heretics, to assign the terms a precise technical meaning. Thus i., distinguished from οὐσία (οὐσία) and from fusis (οὐσία), assumed the proper meaning of an individual subsisting through itself and in itself, that is, of a person, and the Trinitarian dogma received its definitive orthodox formulation: one οὐσία or fusis (= substance or essence or nature) in three i. (= three persons).
Likewise, in stating the mystery of the Incarnation, ambiguities arose unless the meaning of i. was properly specified. For if it was taken in the sense of οὐσία and fusis, one had to say, in order to remain within orthodoxy, that there are two i. in Christ, because saying that there is one i. in Christ affirmed the fusion of the human and divine natures, thus falling into Monophysitism.
If, on the other hand, it was taken in the sense of person, one had to say, in order to be orthodox, that there is one i. in Christ, so as not to fall into Nestorianism.
By specifying instead the meaning of i. as person or an entity subsisting through itself and in itself, the orthodox formula of the mystery of the Incarnation was established as follows: in Christ there is one i. (the divine person, the Word) and two οὐσίαi or fusis (the divine and the human nature); the union, therefore, of the Word with human hypostatic union (v.).