If Catholic Theology does not owe to St. Basil the distinction between the connotations of οὐσία and ὑπόστασις which soon prevailed over the identification obtaining at the time of the Nicene Council, at all events his is the first and most famous assertion and defence of it.  At Nicæa, in 325, to have spoken of St. Paul as “distinguishing the hypostases” would have been held impious.  Some forty-five years later St. Basil writes to his brother, Gregory of Nyssa (Ep. xxxviii.), in fear lest Gregory should fall into the error of failing to distinguish between hypostasis and ousia, between person and essence.  cf. Theodoret Dial. i. 7, and my note on his Ecc. Hist. i. 3.