On the question whether ὺπόστασις and οὐσία have in this passage the same or different meanings, see Bull, Def. Fid. Nic. II. 9, 11, p. 314 (Oxf. Ed.).  Athanasius expressly states that they are perfectly equivalent:  “Subsistence (ὑπόστασις) is essence (οὐσία), and means nothing else but very being, which Jeremiah calls existence (ὕπαρξις).”  Basil distinguishes them, and is followed by Bishop Bull, whose opinion is controverted by Mr. Robertson in an Excursus on the meaning of the phrase, on p. 77 of his edition of Athanasius in this Series.  The student who desires to pursue the subject may consult in addition to the works just named, and the authorities therein mentioned, Dr. Newman’s Arians of the Fourth Century, especially chap. v. sect. i. 3, and Appendix, note iv., on “the terms οὐσία and ὑπόστασις as used in the early Church;” Mr. Robertson’s Prolegomena, ch. ii. § 3 (2) (b); and the Rev. H. A. Wilson’s Prolegomena to Gregory of Nyssa, ch. iv., in this Series.