is explained in Cyril by another term, viz. that of hypostatic union: Cyril teaches a ἕνωσις καθ' ὑπόστασιν. Nestorius, on the contrary, protested against this phrase. In his Treatise of Heraclides he deals much with the question of this phrase and openly says that he did not understand it then (when he first heard it) and did not understand it now[1].
Indeed this term has its difficulties. If we wish to comprehend in which sense Cyril made use of it and Nestorius opposed it, we must, as Professor Bethune-Baker rightly remarks[2], put out of the question that meaning of the term which is taught by the council of Chalcedon and adopted by the orthodoxy of later times, for this meaning is a result of a development, which was not yet completed when Cyril and Nestorius wrote. Originally ὑπόστασις is a synonym of οὐσία, if this latter is understood in the sense of real being; both words then may be translated by substance. As synonymous with οὐσία the term ὑπόστασις appears in the Nicene creed, because the Logos here is deduced ἐκ τῆς οὐσίας τοῦ πατρός and the assertion is anathematised, that he was ἐξ ἑτέρας οὐσίας ἢ ὑποστάσεως. And Athanasius said even about the end of his life: ἡ ὑπόστασις οὐσία ἐστι καὶ οὐδὲν ἄλλο σημαινόμενον ἔχει ἢ αὐτὸ τὸ ὀν[3]. Αὐτὸ τὸ ὄν, the being itself—that is the meaning of ὑπόστασις. The term
