Objection 1. It would seem that the Holy Ghost does not proceed from the Son. For as Dionysius says (Div. Nom. i): "We must not dare to say anything concerning the substantial Divinity except what has been divinely expressed to us by the sacred oracles." But in the Sacred Scripture we are not told that the Holy Ghost proceeds from the Son; but only that He proceeds from the Father, as appears from John 15:26: "The Spirit of truth, Who proceeds from the Father." Therefore the Holy Ghost does not proceed from the Son.
Objection 2. Further, In the creed of the council of Constantinople (Can. vii) we read: "We believe in the Holy Ghost, the Lord and Life-giver, who proceeds from the Father; with the Father and the Son to be adored and glorified." Therefore it should not be added in our Creed that the Holy Ghost proceeds from the Son; and those who added such a thing appear to be worthy of anathema.
Objection 3. Further, Damascene says (De Fide Orth. i): "We say that the Holy Ghost is from the Father, and we name Him the spirit of the Father; but we do not say that the Holy Ghost is from the Son, yet we name Him the Spirit of the Son." Therefore the Holy Ghost does not proceed from the Son.
Objection 4. Further, Nothing proceeds from that wherein it rests. But the Holy Ghost rests in the Son; for it is said in the legend of St. Andrew: "Peace be to you and to all who believe in the one God the Father, and in His only Son our Lord Jesus Christ, and in the one Holy Ghost proceeding from the Father, and abiding in the Son." Therefore the Holy Ghost does not proceed from the Son.
Objection 5. Further, the Son proceeds as the Word. But our breath [spiritus] does not seem to proceed in ourselves from our word. Therefore the Holy Ghost does not proceed from the Son.
Objection 6. Further, the Holy Ghost proceeds perfectly from the Father. Therefore it is superfluous to say that He proceeds from the Son.
Objection 7. Further "the actual and the possible do not differ in things perpetual" (Phys. iii, text 32), and much less so in God. But it is possible for the Holy Ghost to be distinguished from the Son, even if He did not proceed from Him. For Anselm says (De Process. Spir. Sancti, ii): "The Son and the Holy Ghost have their Being from the Father; but each in a different way; one by Birth, the other by Procession, so that they are thus distinct from one another." And further on he says: "For even if for no other reason were the Son and the Holy Ghost distinct, this alone would suffice." Therefore the Holy Spirit is distinct from the Son, without proceeding from Him.
On the contrary, Athanasius says: "The Holy Ghost is from the Father and the Son; not made, nor created, nor begotten, but proceeding."
I answer that, It must be said that the Holy Ghost is from the Son. For if He were not from Him, He could in no wise be personally distinguished from Him; as appears from what has been said above (28, 3; 30, 2). For it cannot be said that the divine Persons are distinguished from each other in any absolute sense; for it would follow that there would not be one essence of the three persons: since everything that is spoken of God in an absolute sense, belongs to the unity of essence. Therefore it must be said that the divine persons are distinguished from each other only by the relations. Now the relations cannot distinguish the persons except forasmuch as they are opposite relations; which appears from the fact that the Father has two relations, by one of which He is related to the Son, and by the other to the Holy Ghost; but these are not opposite relations, and therefore they do not make two persons, but belong only to the one person of the Father. If therefore in the Son and the Holy Ghost there were two relations only, whereby each of them were related to the Father, these relations would not be opposite to each other, as neither would be the two relations whereby the Father is related to them. Hence, as the person of the Father is one, it would follow that the person of the Son and of the Holy Ghost would be one, having two relations opposed to the two relations of the Father. But this is heretical since it destroys the Faith in the Trinity. Therefore the Son and the Holy Ghost must be related to each other by opposite relations. Now there cannot be in God any relations opposed to each other, except relations of origin, as proved above (Question 28, Article 44). And opposite relations of origin are to be understood as of a "principle," and of what is "from the principle." Therefore we must conclude that it is necessary to say that either the Son is from the Holy Ghost; which no one says; or that the Holy Ghost is from the Son, as we confess.
Furthermore, the order of the procession of each one agrees with this conclusion. For it was said above (27, 2,4; 28, 4), that the Son proceeds by the way of the intellect as Word, and the Holy Ghost by way of the will as Love. Now love must proceed from a word. For we do not love anything unless we apprehend it by a mental conception. Hence also in this way it is manifest that the Holy Ghost proceeds from the Son.
We derive a knowledge of the same truth from the very order of nature itself. For we nowhere find that several things proceed from one without order except in those which differ only by their matter; as for instance one smith produces many knives distinct from each other materially, with no order to each other; whereas in things in which there is not only a material distinction we always find that some order exists in the multitude produced. Hence also in the order of creatures produced, the beauty of the divine wisdom is displayed. So if from the one Person of the Father, two persons proceed, the Son and the Holy Ghost, there must be some order between them. Nor can any other be assigned except the order of their nature, whereby one is from the other. Therefore it cannot be said that the Son and the Holy Ghost proceed from the Father in such a way as that neither of them proceeds from the other, unless we admit in them a material distinction; which is impossible.
Hence also the Greeks themselves recognize that the procession of the Holy Ghost has some order to the Son. For they grant that the Holy Ghost is the Spirit "of the Son"; and that He is from the Father "through the Son." Some of them are said also to concede that "He is from the Son"; or that "He flows from the Son," but not that He proceeds; which seems to come from ignorance or obstinacy. For a just consideration of the truth will convince anyone that the word procession is the one most commonly applied to all that denotes origin of any kind. For we use the term to describe any kind of origin; as when we say that a line proceeds from a point, a ray from the sun, a stream from a source, and likewise in everything else. Hence, granted that the Holy Ghost originates in any way from the Son, we can conclude that the Holy Ghost proceeds from the Son.
Reply to Objection 1. We ought not to say about God anything which is not found in Holy Scripture either explicitly or implicitly. But although we do not find it verbally expressed in Holy Scripture that the Holy Ghost proceeds from the Son, still we do find it in the sense of Scripture, especially where the Son says, speaking of the Holy Ghost, "He will glorify Me, because He shall receive of Mine" (John 16:14). It is also a rule of Holy Scripture that whatever is said of the Father, applies to the Son, although there be added an exclusive term; except only as regards what belongs to the opposite relations, whereby the Father and the Son are distinguished from each other. For when the Lord says, "No one knoweth the Son, but the Father," the idea of the Son knowing Himself is not excluded. So therefore when we say that the Holy Ghost proceeds from the Father, even though it be added that He proceeds from the Father alone, the Son would not thereby be at all excluded; because as regards being the principle of the Holy Ghost, the Father and the Son are not opposed to each other, but only as regards the fact that one is the Father, and the other is the Son.
Reply to Objection 2. In every council of the Church a symbol of faith has been drawn up to meet some prevalent error condemned in the council at that time. Hence subsequent councils are not to be described as making a new symbol of faith; but what was implicitly contained in the first symbol was explained by some addition directed against rising heresies. Hence in the decision of the council of Chalcedon it is declared that those who were congregated together in the council of Constantinople, handed down the doctrine about the Holy Ghost, not implying that there was anything wanting in the doctrine of their predecessors who had gathered together at Nicaea, but explaining what those fathers had understood of the matter. Therefore, because at the time of the ancient councils the error of those who said that the Holy Ghost did not proceed from the Son had not arisen, it was not necessary to make any explicit declaration on that point; whereas, later on, when certain errors rose up, another council [Council of Rome, under Pope Damasus] assembled in the west, the matter was explicitly defined by the authority of the Roman Pontiff, by whose authority also the ancient councils were summoned and confirmed. Nevertheless the truth was contained implicitly in the belief that the Holy Ghost proceeds from the Father.
Reply to Objection 3. The Nestorians were the first to introduce the error that the Holy Ghost did not proceed from the Son, as appears in a Nestorian creed condemned in the council of Ephesus. This error was embraced by Theodoric the Nestorian, and several others after him, among whom was also Damascene. Hence, in that point his opinion is not to be held. Although, too, it has been asserted by some that while Damascene did not confess that the Holy Ghost was from the Son, neither do those words of his express a denial thereof.
Reply to Objection 4. When the Holy Ghost is said to rest or abide in the Son, it does not mean that He does not proceed from Him; for the Son also is said to abide in the Father, although He proceeds from the Father. Also the Holy Ghost is said to rest in the Son as the love of the lover abides in the beloved; or in reference to the human nature of Christ, by reason of what is written: "On whom thou shalt see the Spirit descending and remaining upon Him, He it is who baptizes" (John 1:33).
Reply to Objection 5. The Word in God is not taken after the similitude of the vocal word, whence the breath [spiritus] does not proceed; for it would then be only metaphorical; but after the similitude of the mental word, whence proceeds love.
Reply to Objection 6. For the reason that the Holy Ghost proceeds from the Father perfectly, not only is it not superfluous to say He proceeds from the Son, but rather it is absolutely necessary. Forasmuch as one power belongs to the Father and the Son; and because whatever is from the Father, must be from the Son unless it be opposed to the property of filiation; for the Son is not from Himself, although He is from the Father.
Reply to Objection 7. The Holy Ghost is distinguished from the Son, inasmuch as the origin of one is distinguished from the origin of the other; but the difference itself of origin comes from the fact that the Son is only from the Father, whereas the Holy Ghost is from the Father and the Son; for otherwise the processions would not be distinguished from each other, as explained above, and in 27.
Source: Summa Theologica (New Advent)