Translated by W. Moore and H. A. Wilson
To Eustathius  .
All you who study medicine have, one may say, humanity for your profession: and I think that one who preferred your science to all the serious pursuits of life would form the proper judgment, and not miss the right decision, if it be true that life, the most valued of all things, is a thing to be shunned, and full of pain, if it may not be had with health, and health your art supplies. But in your own case the science is in a notable degree of double efficacy; you enlarge for yourself the bounds of its humanity, since you do not limit the benefit of your art to men's bodies, but take thought also for the cure of troubles of the mind. I say this, not only following the common reports, but because I have learnt it from experience, as in many other matters, so especially at this time in this indescribable malice of our enemies, which you skilfully dispersed when it swept like some evil flood over our life, dispelling this violent inflammation of our heart by your fomentation of soothing words. I thought it right, indeed, in view of the continuous and varied effort of our enemies against us, to keep silence, and to receive their attack quietly, rather than to speak against men armed with falsehood, that most mischievous weapon, which sometimes drives its point even through truth. But you did well in urging me not to betray the truth, but to refute the slanderers, lest, by a success of falsehood against truth, many might be injured.
I may say that those who conceived this causeless hatred for us seemed to be acting very much on the principle of aesop's fable. For just as he makes his wolf bring some charges against the lamb (feeling ashamed, I suppose, of seeming to destroy, without just pretext, one who had done him no hurt), and then, when the lamb easily swept away all the slanderous charges brought against him, makes the wolf by no means slacken his attack, but carry the day with his teeth when he is vanquished by justice; so those who were as keen for hatred against us as if it were something good (feeling perhaps some shame of seeming to hate without cause), make up charges and complaints against us, while they do not abide consistently by any of the things they say, but allege, now that one thing, after a little while that another, and then again that something else is the cause of their hostility to us. Their malice does not take a stand on any ground, but when they are dislodged from one charge they cling to another, and from that again they seize upon a third, and if all their charges are refuted they do not give up their hate. They charge us with preaching three Gods, and din into the ears of the multitude this slander, which they never rest from maintaining persuasively. Then truth fights on our side, for we show both publicly to all men, and privately to those who converse with us, that we anathematize any man who says that there are three Gods, and hold him to be not even a Christian. Then, as soon as they hear this, they find Sabellius a handy weapon against us, and the plague that he spread is the subject of continual attacks upon us. Once more, we oppose to this assault our wonted armour of truth, and show that we abhor this form of heresy just as much as Judaism. What then? are they weary after such efforts, and content to rest? Not at all. Now they charge us with innovation, and frame their complaint against us in this way:--They allege that while we confess  three Persons we say that there is one goodness, and one power, and one Godhead. And in this assertion they do not go beyond the truth; for we do say so. But the ground of their complaint is that their custom does not admit this, and Scripture does not support it. What then is our reply? We do not think that it is right to make their prevailing custom the law and rule of sound doctrine. For if custom is to avail for  proof of soundness, we too, surely, may advance our prevailing custom; and if they reject this, we are surely not bound to follow theirs. Let the inspired Scripture, then, be our umpire, and the vote of truth will surely be given to those whose dogmas are found to agree with the Divine words.
 The greater part of this treatise is found also among the Letters of S. Basil [Ep. 189 (80): Ed. Gaume, Tom iii. p. 401 (276 c.)]. The Benedictine edition of S. Basil notes that in one ms. a marginal note attributes the letter to Gregory. It may be added that those parts which appear to be found only in the mss. of Gregory make the argument considerably clearer than it is if they are excluded, as they are from the Benedictine text of S. Basil.
 Reading homologountas with Oehler. The Paris Edit. reads homologounton, and so also the Benedictine S. Basil. The Latin translator of 1615, however, renders as if he had read homologountas
 Reading eis orthotetos apodeixin, with Oehler and the Benedictine S. Basil. The Paris Edit. of 1615 reads eis orthoteta logou.
Reference address : https://www.elpenor.org/nyssa/holy-trinity-spirit.asp