Most students of the filioque controversy will be familiar with the Council of Blachernae and the associated Tome written by Gregory of Cyprus. Gregory articulates a distinction between the Holy Spirit "having existence from" the Father and "existing through" the Son, wherein the latter is described as "manifestation." The standard account, offered by Aristeides Papadakis in Crisis in Byzantium, is that this refers to the eternal manifestation of the divine energies, the activities of the divinity. One might also think about this view as the consubstantial sharing of the divine essence.
My thinking on the subject of eternal manifestation has been changed by an article by Anne-Sophie Vivier-Muresan entitled "The eternal manifestation of the Spirit through the Son: a hypostatic or energetic reality? Inquiry in the works of Gregory of Cyprus and Gregory Palamas." In that article, Vivier-Muresan notes that while there are clear similarities between the idea of energies as expressed in the economy and eternal manifestation, the concepts are not used identically. But if eternal manifestation is not eternal energetic procession, what might it be?
One of the earliest sources appears to be the creed of St. Gregory Thaumaturgus, reproduced by St. Gregory of Nyssa in his biography of the saint. It includes the following statement on the Holy Spirit:
Ἑν πνεῦμα ἅγιον, ἐκ θεοῦ τὴν ὕπαρξιν ἔχον, καὶ διὰ υἱοῦ πεφηνὸς, εἰκὼν τοῦ υἱοῦ τελείου τελεία, ζωὴ ζώντων αἰτία, ἀγιότης ἀγιασμοῦ χορηγός, ἐν ᾧ φανεροῦται θεὸς ὁ πατήρ, ὁ ἐπὶ πάντων καὶ ἐν πᾶσι, καὶ θεὸς ὁ υἱός, ὁ διὰ πάντων.
One Holy Spirit, having substantial existence from God, manifested through the Son, perfect image of the perfect Son, living cause of living things, sanctity and provider of sanctification, by whom God the Father is manifested, who is over all and in all, and God the Son, who is through all.
I believe that "having substantial existence from" and "manifested through" can be analogized to the efficient causal relations of efficient originating cause (causa secundum fieri, cause of becoming) and efficient sustaining cause (causa secundum esse, cause of being). Specifically, the Father is the originating cause of both the Son and the Spirit, the Father is the sustaining cause of the Son, and the Father and the Son as one principle (qua consubstantial) are the sustaining cause of the Spirit.
St. Thomas explains the distinction between these two causes as follows (ST I, 104, 1, ans.):
Every effect depends on its cause, so far as it is its cause. But we must observe that an agent may be the cause of the "becoming" [causa secundum fieri, originating cause] of its effect, but not directly of its "being" [causa secundum esse, sustaining cause]. This may be seen both in artificial and in natural beings: for the builder causes the house in its "becoming," but he is not the direct cause of its "being." For it is clear that the "being" of the house is a result of its form, which consists in the putting together and arrangement of the materials, and results from the natural qualities of certain things. Thus a cook dresses the food by applying the natural activity of fire; thus a builder constructs a house, by making use of cement, stones, and wood which are able to be put together in a certain order and to preserve it. Therefore the "being" of a house depends on the nature of these materials, just as its "becoming" depends on the action of the builder. The same principle applies to natural things. For if an agent is not the cause of a form as such, neither will it be directly the cause of "being" which results from that form; but it will be the cause of the effect, in its "becoming" only.
To prevent misunderstanding of what I am saying, I am not suggesting that the Holy Spirit is the sort of being that would require a sustaining cause. God is not liable to corruption and therefore requires no efficient cause to prevent corruption. (For that reason, St. Thomas himself would never have had a reason to apply this concept in this way.) Nor am I suggesting that the Persons of the Trinity are effects of some causal power of the Father, at least in the ordinary way we would think about causality. St. Thomas himself warns that thinking of efficient causality in the normal sense of producing an exterior effect is liable to lead to Arianism or Sabellianism (ST I, 27, 1, ans.). Lastly, I am not suggesting that there is some sort of temporal succession between the Spirit "beginning to exist" and existing, nor is there even a moment when the Spirit began to exist. Rather, I am suggesting that if the concept is suitably modified in the way that we ordinarily do when speaking of causality in the immanent Trinity, it provides insight into what distinction Gregory (and other Eastern authors) could have had in mind when writing about eternal manifestation.
Importantly, the Greek authors themselves did not express or articulate the concept in this way. Gregory's interlocutor John Bekkos seems completely baffled as to how this distinction could be coherent. Peter Gilbert
has helpfully translated his objection as follows:
First, as for what he says concerning the eternal manifestation through the Son, I am unable to discern if the eternal shining-forth of the Spirit through the Son means anything different than his eternal existence through the Son. Next, even if I should admit a distinction between these terms, I do not regard them as opposed in the way he says they are, in keeping with his own view. For since the chief end he has in view is the unmediated existence of the Spirit from the Father, if the shining-forth of the Spirit through the Son is something other than his existence through the Son — for example, his being bestowed upon us through the Son — then why would the Holy Spirit be thought to shine forth through the Son eternally, if it is not the case that, through the Son, he has existence?
With originating and sustaining cause, we have a causal relation concerning the existence of the effect that nevertheless makes a distinction in the manner of causation. If we consider the Holy Spirit's existence as effect of an efficient cause, we can make a logical distinction between the cause of the Holy Spirit's beginning-to-exist and the cause of the Holy Spirit's remaining-in-existence. The Father alone is the originating cause, and the Father and the Son together, as one principle, are the sustaining cause. Both causes are, in some aspect, cause of the same existence, but they operate in a different manner in doing so.
Of course, this application was not considered at the time. The Greek use of aitia would have corresponded only to my usage of efficient originating cause. Due to the eternal and unchanging nature of God and the absence of an exterior effect, there would have been no natural reason to connect that causal concept to kinoun, the mover, the efficient cause. But if we modify the concept in an appropriate way for ad intra processions, including interiority and eternity, there is no reason that the analogy cannot apply. If we introduce the distinction between originating and sustaining cause, then the originating cause accounts for what was already discussed explicitly about causality of the divine Persons, while the sustaining cause is implicitly behind the distinction between "having existence from" and "existing through." Furthermore, if that distinction maps back to the distinction between "having substantial existence from" and "manifesting through" offered in the creed of Gregory of Neocaesarea, then it can also account for similar expressions found in St. Gregory of Nyssa, who uses "manifesting through" explicitly, and St. Cyril of Alexandria, who uses "substantially from the Father and the Son" and "from the Son's essence." Perhaps most importantly, it would account for a highly controversial passage cited at Blachernae from St. John Damascene: "He Himself [the Father], then, is mind, the depth of reason, begetter of the Word, and, through the Word, projector of the manifesting Spirit." (Emphasis added; the citation from Papadakis at p. 229 is to Kotter's translation of the Exact Exposition of the Orthodox Faith.)
As I said, there is no reason that this concept would have been applied in this way in the West. Nonetheless, I do not see any incompatibility when it is suitably modified, since the Latin view all the way back to St. Augustine espouses a difference between the Father as principle-without-principle and the Son as principle-from-principle. St. Augustine likewise distinguishes between the Spirit taking His origin from the Father principaliter and from the Father and the Son in communion (communiter). The distinction between originating cause and sustaining cause provides a suitable explanation. But originating and sustaining causes can be different things, as with the builder and the form of the building or the parent and the soul of the offspring. For that reason, retaining the "as from one principle" concept is essential to maintaining the identity between originating and sustaining causes, so that they are not two causes. This unity with respect to efficient causality has been affirmed by Bl. Duns Scotus, who maintains that the Father and the Son are not two causes acting in concert (like two oxen drawing a cart), but rather act "as one agent" (Richard Cross, Duns Scotus on God, p. 216). The same consubstantial unity that assures the Son and the Father act as one agent, which we can take as one efficient cause for purposes of the analogy, assures that there is identity between the Father as originating cause and the Father and the Son consubstantially (communiter) as sustaining cause.
In short, the distinction between efficient originating cause and efficient sustaining cause can account for the distinction between "having existence from" and "existing through" in the East and "principle without principle"/principaliter and "principle-from-principle"/communiter in the West. Even though this distinction was not explicitly deployed in either side (nor would it have been for the reasons that I laid out previously), I see no reason that it cannot serve as an interpretive lens today. The view provided by that lens seems both clear and accurate.