3. My statement, then, is this. That which is spoken of in a special and peculiar manner is indicated by the name of the hypostasis. Suppose we say “a man.” The indefinite meaning of the word strikes a certain vague sense upon the ears. The nature is indicated, but what subsists and is specially and peculiarly indicated by the name is not made plain. Suppose we say “Paul.” We set forth, by what is indicated by the name, the nature subsisting.
This then is the hypostasis, or “understanding;” not the indefinite conception of the essence or substance, which, because what is signified is general, finds no “standing,” but the conception which by means of the expressed peculiarities gives standing and circumscription to the general and uncircumscribed. It is customary in Scripture to make a distinction of this kind, as well in many other passages as in the History of Job. When purposing to narrate the events of his life, Job first mentions the common, and says “a man;” then he straightway particularizes by adding “a certain.” As to the description of the essence, as having no bearing on the scope of his work, he is silent, but by means of particular notes of identity, mentioning the place and points of character, and such external qualifications as would individualize, and separate from the common and general idea, he specifies the “certain man,” in such a way that from name, place, mental qualities, and outside circumstances, the description of the man whose life is being narrated is made in all particulars perfectly clear. If he had been giving an account of the essence, there would not in his explanation of the nature have been any mention of these matters. The same moreover would have been the account that there is in the case of Bildad the Shuhite, and Zophar the Naamathite, and each of the men there mentioned. Transfer, then, to the divine dogmas the same standard of difference which you recognise in the case both of essence and of hypostasis in human affairs, and you will not go wrong.
Letter XXXVIII of St. Basil To his Brother Gregory, concerning the difference between οὐσία and ὑπόστασις. NPNF s. II, vol. 8, pp. 137-138.
Footnote 2022 of the NPNF series states this in regard to this Letter:
This important letter is included as among the works of Gregory of Nyssa, as addressed to Peter, bp. of Sebaste, brotherof Basil and Gregory. The Ben. note says: “Stylus Basilii fetum esse clamitat.” It was moreover, referred to at Chalcedon as Basil’s. [Mansi, T. vii. col. 464.]
However, modern scholarship now states that this is in fact a letter penned by St. Gregory of Nyssa. Perhaps it is to another Gregory or to even a budding catechumen or theologian. See the discussions by Fedwich and Cavallin as they document the scholars who attribute this letter to Gregory of Nyssa. For our discussion, whoever of the two Cappadocian brothers wrote this adds little to our concern since the practice of this principle is undoubtedly found in both.
 ὑφεστῶσαν. & 195·πόστασις is derivatively that which “stands under” or subsists, ὃ ὑφέστηκε. cf. my note on Theodoret, p. 36.
 Job i. 1, LXX.
 Job ii. 11.