means[1] τὸ ὑποκείμενον, as Aristotle said, the ultimate reality which is the bearer of all the attributes which are called the nature of a thing, the substance in the sense in which the earlier philosophy, that of the middle ages included, made use of this term and which was afterwards criticised by Locke and Hume. The term οὐσία could also be used in a generic sense and then received a meaning similar to kind or nature, but ὑπόστασις means only that which οὐσία could mean in addition to its other meaning, viz., a single and really existing being, whether material or immaterial.
As regards the doctrine of the Trinity these two terms, originally synonymous to some extent, were differentiated: one spoke of μία οὐσία and τρεῖς ὑποστάσεις in the Trinity; but, as Professor Bethune-Baker rightly observed[2], there is not any clear evidence that a similar usage, a similar differentiation between οὐσία and ὑπόστασις, had been extended in the time of Cyril to the christological problem. Hence in the discussion between Cyril and Nestorius on the relation of the Godhead and manhood in Christ the term ὑπόστασις must be understood as essentially synonymous with οὐσία. Now Nestorius, just as the earlier Antiochians, believed that the natures of Christ, as both really existing in him, had each their ὑπόστασις: he spoke of two ὑποστάσεις with as little scruple as of two
