The difference between Generation and Spiration is not precisely understood by us. AUGUSTINE, in book II or III contra Maximinum, chapter XIV, opera, tome 8, column 489, modestly: “But what the difference is between to be begotten and to proceed, speaking of that infinitely exalted nature, who is able to explain? Not everything that proceeds is begotten, although everything that is begotten proceeds; just as not everthing that is bipedal is a man, although everthing that is a man is bipedal. These things I know: but to distinguish between that Generation and this Procession I do not know, am not able, am not sufficient, because both the former and the latter are ineffable.” Thus also DAMASCENUS, book I, Concerning the Orthodox Faith, chapter X, Ὅτι μὲν ἐστὶ διαφορὰ γεννήσεως καὶ ἐκπορεύσεως μεμαθήκαμεν· τίς δὲ ὁ τρόπος τῆς διαφορᾶς, οὐδαμῶς, We have understood that there is a difference between generation and procession: but what the manner of the difference is, in no wise: compare LAMPE’S more lengthy treatement in Dissertationum philologico-theologicarum, volume II, Disputation VI, chapter VI, de Spiritu Sancto, § 26, 30-37, pages 212, 215-224.
Nevertheless, this is evident, that in Generation the faculty of further communication of Essence is imparted to the Son, but not likewise in Spiration to the Holy Spirit. And in this especially it is permissible for us to conceive that Generation and Spiration are distinct. Moreover, we are able to add: 1. That Going Forth is indeed attributed to both the Son and the Spirit; but to the Son alone through the mode of Generation and Birth, to the Spirit alone through the mode of Spiration as breath from the mouth of the Almighty. PETER MARTYR,Locis communibus, class I, chapter XI, page 71: “Although it is difficult to specify the distinction between Procession and Generation, and Augustine, in the passage now adduced, confesses himself to be ignorant of it: yet he does say that he knows this, that what is begotten also proceeds: but it is not so in turn, that thing that proceed are generated. But we are not able to express the proper distinction. And so the Holy Spirit is called neither begotten, nor unbegotten: lest, if we should call Him unbegotten, we appear to make Him the Father, and, if we should establish Him as begotten, we appear to call Him the Son. This is taken from Augustine, etc. You might add: But if the Holy Spirit be called begotten, then in the Trinity two Sons would be constituted, and two Fathers. For, since the Holy Spirit is both from the Father and from the Son, He would have both as parents, if He be said to be begotten by them. Indeed, if the matter be rightly weighed, then He would be called both the son and the grandson of the Father. For, insofar as He is alleged to be generated by the Father, He would be called His Son: but, insofar as He is said to be born of the Son, He would be the grandson of the Father: which things are absurd, and foreign to the Scriptures. Then, when the Holy Spirit is called begotten, the words of Scripture object, which frequently call the Son Only Begotten: from which it follows that the Holy Spirit is not begotten, John 1:14; 3:16; 1 John 4:9. And Christ, with respect to His human nature, is not wont to be called in the sacred books the Only Begotten of God, but rather the Firstborn among many brethren, as it is evident in Romans 8:29; but with respect to the divine nature, He has no brothers.” 2. That Generation proceeds from the Father alone, but Spiration from the Father and the Son, or from the Father through the Son. 3. And that, just as the Son is the second Person of the Trinity, and the Holy Spirit is the third; so Generation according to our mode of conception precedes Spiration, although they are verily co-eternal.
But, that with the Scholastics many Theologians attribute Generation to the Intellect and Spiration to the Will, is vain. That is, in this way the Scholastics, by probing this mystery with over much subtlety, have verily hallucinated, divining that this is the reason why the Son is called Wisdom and Λόγος/Logos/Word, but the Holy Spirit goes under the name of Love. While, 1. in chapter I, § 3, and chapter IV, § 34, it was seen that sufficient reasons are given why the Son might be called the Λόγος/ Logos/Word and Wisdom of God, although it is not on account of conceiving of His Generation through the mode of Understanding, or as an act proceding from the divine Intellect. 2. They gratuitously suppose that the Spirit is called Love, who more accurately is set forth to us as a Witness and Token of divine Love, Romans 5:5. 3. In this way they entangle the matter further, since Generation and Spiration are personal Operations; but to understand and to will are rather essential acts. Whence Ahmed the Persian, a Mohammedan by profession, from this Scholastic explanation of Generationa and Procession, acutely, and not unhappily, impugns the mystery of the Trinity, by arguing in this way: Knowledge and Love in God, the former an act of the intellect, as it were, the latter of the will, are essential; therefore, not the foundation of a personal relation. Likewise, if by an act of the intellect in the divine the Son is produced, then either by the whole understanding and knowledge in God, or not: if the former, then also by the intellect of the Son Himself also as Λόγος/Logos/Word: if by some only, then by another knowledge another Son could also be produced, and so on infinitely: see HOORNBEECK’S Summam Controversiarum, book III, page 136, and likewise page 854; LAMPE’S Dissertationum philologico-theologicarum, volume II, Disputation IV, chapter III, de Doctrina Spiritus Sancti, § 7, pages 128, 129, § 11, 12, pages 132-134, § 29, 30, pages 143-145, Disputation VI, chapter VII, § 23-25, pages 238-240. ERASMUS, Præfatione ad Hilarium, α. 3 versa: Finally, the matter has proceeded to impious audacity…. Under what pretext will we ask pardon for ourselves, who move so many curious, let me not say impious, questions concerning matters so far removed from our nature; who define so many things, that without loss of salvation either are able not to be known, or are able to be left ambiguous? Is he going to have a partnership with the Father, the Son, and the Holy Spirit, who does not know how, according to the account of philosophy, to explain how he might distinguish the Father from the Son, and the Holy Spirit from both, how the nativity of the Son from the Father differs from the procession of the Spirit? If I believe what has been delivered, that the three are of one nature, what is the need of laborious disputation? If I do not believe, persuasion is not going to come by any human reasonings. And dangerous curiosity of this sort has generally been born to us by the study of Philosophy.
 Peter Martyr Vermigli (1499-1562) began his career as an Augustinian monk, preacher, and lecturer in Italy. Through personal study of the Scripture and the Reformers, he came to embrace the Protestant doctrines. He settled in England and served as Professor of Divinity at Oxford and as Canon of Christ Church. Unhappily, he was forced to flee from England as well, when Mary Tudor took the throne. He settled in Zurich and became Professor of Divinity there.