Translated by Bl. Jackson.
This Part: 129 Pages
3. Consider well, my excellent friend, that the falsifiers of the truth, who have introduced the Arian schism as an innovation on the sound faith of the Fathers, advance no other reason for refusing to accept the pious opinion of the Fathers than the meaning of the homoousion which they hold in their wickedness, and to the slander of the whole faith, alleging our contention to be that the Son is consubstantial in hypostasis. If we give them any opportunity by our being carried away by men who propound these sentiments and their like, rather from simplicity than from malevolence, there is nothing to prevent our giving them an unanswerable ground of argument against ourselves and confirming the heresy of those whose one end is in all their utterances about the Church, not so much to establish their own position as to calumniate mine. What more serious calumny could there be? What better calculated to disturb the faith of the majority than that some of us could be shewn to assert that there is one hypostasis of Father, Son, and Holy Ghost? We distinctly lay down that there is a difference of Persons; but this statement was anticipated by Sabellius, who affirms that God is one by hypostasis, but is described by Scripture in different Persons, according to the requirements of each individual case; sometimes under the name of Father, when there is occasion for this Person; sometimes under the name of Son when there is a descent to human interests or any of the operations of the oeconomy;  and sometimes under the Person of Spirit when the occasion demands such phraseology. If, then, any among us are shewn to assert that Father, Son and Holy Ghost are one in substance,  while we maintain the three perfect Persons, how shall we escape giving clear and incontrovertible proof of the truth of what is being asserted about us?
4. The non-identity of hypostasis and ousia is, I take it, suggested even by our western brethren, where, from a suspicion of the inadequacy of their own language, they have given the word ousia in the Greek, to the end that any possible difference of meaning might be preserved in the clear and unconfounded distinction of terms. If you ask me to state shortly my own view, I shall state that ousia has the same relation to hypostasis as the common has to the particular. Every one of us both shares in existence by the common term of essence (ousia) and by his own properties is such an one and such an one. In the same manner, in the matter in question, the term ousia is common, like goodness, or Godhead, or any similar attribute; while hypostasis is contemplated in the special property of Fatherhood, Sonship, or the power to sanctify. If then they describe the Persons as being without hypostasis,  the statement is per se absurd; but if they concede that the Persons exist in real hypostasis, as they acknowledge, let them so reckon them that the principle of the homoousion may be preserved in the unity of the Godhead, and that the doctrine preached may be the recognition of true religion, of Father, Son, and Holy Ghost, in the perfect and complete hypostasis of each of the Persons named. Nevertheless, there is one point which I should like to have pressed on your excellency, that you and all who like you care for the truth, and honour the combatant in the cause of true religion, ought to wait for the lead to be taken in bringing about this union and peace by the foremost authorities in the Church, whom I count as pillars and foundations of the truth and of the Church, and reverence all the more because they have been sent away for punishment, and have been exiled far from home. Keep yourself, I implore you, clear of prejudice, that in you, whom God has given me as a staff and support in all things, I may be able to find rest. 
 Vide notes, pp. 7 and 12. On Sabellius, cf. note on Letter ccxxxvi.
 to hupokeimenon.
 On the point treated of in this letter, cf. note on p. 5 and Letter xxxviii. p. 137. But in the De S.S. cap. 38 (p. 23) St. Basil himself repudiates the assertion of three "original hypostases," when he is apparently using hupostasis in the Nicene sense.
Reference address : https://www.elpenor.org/basil/letters-3.asp