As I've said before, it is not my purpose to offer greater insight into the inner life of God than the Church has enjoyed before. Such an ambition would be absurdly hubristic. My aim is only to reconcile the ways of speaking that are normative in West and East respectively. But that cannot be done just by repeating old affirmations and seeking to draw out their deductive implications: that method has been tried many times before, even at general councils, to no lasting effect. The real challenge is to construe certain premises that both sides either already accept or can accept as legitimate theological proposals, so as to get a helpful result both sides can live with. In my view and that of some theologians from both sides, that requires a new affirmation which, without adding anything to the deposit of faith, would constitute an authentic instance of doctrinal development. Such an affirmation is actually on offer; but I shall save its specific form for the end.
Since most of the objections seem to me to be missing the point, I shall only address what seem to me to be the two most important. To that end, I point out that the Vatican itself is on record as proposing an idea logically equivalent to mine.
In a much-discussed 1995 white paper from the Pontifical Council for Promoting Christian Unity, we read (emphasis added):
If it is correctly situated, the Filioque of the Latin tradition must not lead to a subordination of the Holy Spirit in the Trinity. Even if the Catholic doctrine affirms that the Holy Spirit proceeds from the Father and the Son in the communication of their consubstantial communion, it nonetheless recognizes the reality of the original relationship of the Holy Spirit as person with the Father, a relationship that the Greek Fathers express by the term ekporeusis.5
In the same way, if in the Trinitarian order the Holy Spirit is consecutive to the relation between the Father and the Son, since he takes his origin from the Father as Father of the only Son,6 it is in the Spirit that this relationship between the Father and the Son itself attains its Trinitarian perfection. Just as the Father is characterized as Father by the Son he generates, so does the Spirit, by taking his origin from the Father, characterize the Father in the manner of the Trinity in relation to the Son and characterizes the Son in the manner of the Trinity in his relation to the Father: in the fullness of the Trinitarian mystery they are Father and Son in the Holy Spirit.7
The Father only generates the Son by breathing (proballein in Greek) through him the Holy Spirit and the Son is only begotten by the Father insofar as the spiration (probolle in Greek) passes through him. The Father is Father of the One Son only by being for him and through him the origin of the Holy Spirit.8
The Spirit does not precede the Son, since the Son characterizes as Father the Father from whom the Spirit takes his origin, according to the Trinitarian order.9 But the spiration of the Spirit from the Father takes place by and through (the two senses of dia in Greek) the generation of the Son, to which it gives its Trinitarian character. It is in this sense that St John Damascene says: "The Holy Spirit is a substantial power contemplated in his own distinct hypostasis, who proceeds from the Father and reposes in the Word" (De Fide orthodoxa I, 7, PG 94, 805 B, ed. B. Kotter, Berlin 1973, p. 16; Dialogus contra Manichaeos 5, PG 94, 1512 B, ed. B. Kotter, Berlin 1981, p. 354).10Though inspired by St. Gregory of Nazianzus, the highlighted statement does not have magisterial force, for it was not made by an organ of the magisterium. Neither has it been the explicit and ordinary teaching of the Church in the past; rather, I take its appearance in the document as a proposal for authentic doctrinal development. From that standpoint, the mere fact that a Vatican commission has attempted to advance discussion by such means, and has not in any way been reigned in for it, shows that Rome sees the proposal as within the ambit of Trinitarian orthodoxy.
Assuming it is orthodox to say that the Father begets the Son only "by" breathing forth the Holy Spirit "through" the Son (whatever 'through' may mean here, which is a distinct question), then there is a sense in which we may say that the Son "comes forth" or "proceeds" ex Patri spirituque, i.e. from the Father and the Holy Spirit. That does not mean, nor should it mean, that the Holy Spirit originates the Son in the same way as the Father, as if the two could be said to act in exactly the same way in producing the Holy Spirit—any more than the Holy Spirit's coming forth or proceeding from the Father and the Son does or should mean that the Son originates the Holy Spirit in the same sort of way as the Father, in the sense of dual procession. It means that the Father and the Holy Spirit beget the Son "as from one principle," that principle being the Father as spirator, just as the Father and the Son breathe forth the Holy Spirit "as from one principle", that principle being the Father as Father of the Son. Besides reaffirming the monarchy of the Father, that sense of "as from one principle" permits and calls for saying that the Father originates the Son and the Holy Spirit only in some sort of relation to each other.
Getting clear about that is essential for my purpose, since the most serious criticism of my proposal was that it is incompatible with certain conciliar affirmations. Thus, Robert Kovacs pointed out that, in the bull of union with the Copts (1442) promulgated by Pope Eugene IV, it is professed as the faith of "the holy Roman church" that "the Father alone from his substance begot the Son; the Son alone is begotten of the Father alone; the Holy Spirit alone proceeds at once from the Father and the Son" (emphasis added). But that is only to say that "begetting" is what the Father does to originate the Son as distinct from what he does to originate the Holy Spirit. It is not to say, nor does it entail, that the Father's origination of the Son by "begetting" has nothing to do with the Father's origination of the Spirit by "breathing forth." And the same considerations apply to other conciliar statements Robert quotes; otherwise, the statement I've highlighted from the Vatican white paper would be heretical, which cannot reasonably said.
The other major objection to spirituque that I need to consider was registered by Brandon Watson:
I can understand saying that the Son is related to the Father and the Spirit, so if that's all you mean, I agree; but this is not the same as saying that the Son is begotten from the Father and the Spirit, which is what seems implied by the word itself. Everyone must admit the former; the latter seems dubious, particularly if we affirm the Filioque, because the Filioque restricts what can be said about the Son's generation by identifying a distinctive characteristic of the Spirit that must be preserved in distinguishing the other Two. The Son's relationship to the Spirit must reciprocate the Spirit's relationship to the Son, not run parallel to it. Otherwise we seem not to have done anything to distinguish the Spirit's procession from the Son's begetting.
The force of that objection is clear: to affirm spirituque is obliterate the distinction between how the Son is originated and how the Spirit is originated. But as Brandon suspects, I think that is "sticking at words." Construed as I've been explaining, spirituque is an indirect logical consequence of what the Vatican statement affirms, i.e., that the Father begets the Son "by" breathing forth the Holy Spirit "through" him. Given that statement, it can also be said that the Father begets the Son "in" the Holy Spirit; from that, it follows that the Son is begotten by the Father "and" the Spirit as from one principle. It must not of course be said that the Spirit begets the Son by the same activity as the Father, any more than it may be said that the Son breathes forth the Spirit by the same activity as the Father; saying either would be incompatible with the monarchy of the Father. What can and, I believe, should be said is that the Father's origination of the Son only takes place in virtue of the Spirit's, just as the Father's origination of the Spirit only takes place in virtue of the Son's. The latter gives us an orthodox sense of filioque; the former gives us an orthodox sense of spirituque. That kind of parallelism does not preclude the kind of reciprocal relation that Brandon rightly notes, for what is denoted by 'in virtue of' differs in the two cases, and differs in a way that preserves whatever other relation obtains between the Son and the Spirit specifically . So it does not run afoul of his objection.
For the position I'm defending, I am much indebted to a paper I'm fond of recommending: Fr. Thomas Weinandy, OFM Cap., “Clarifying the Filioque: The Catholic-Orthodox Dialogue,” Communio 23, no. 2 (1996): 354-367 K. Then lecturing at Oxford, Weinandy wrote that paper in reaction to the Vatican document and as a followup to his book The Father's Spirit of Sonship: Reconceiving the Trinity; currently he heads the USCCB's Committee on Doctrine. As an ecumenical proposal, Weinandy concludes his paper with the following suggested reformulation of the Creed:
We believe in the Holy Spirit, the Lord the Giver of Life, who comes forth (ejkporeuvetai) from the Father as the one in whom the Son is begotten and who proceeds (proei'si) from and through the Son in communion with the Father, and together with the Father and the Son is worship and glorified.
That's the "new affirmation" I believe would signify having got beyond the filioque impasse. Perhaps that's what we should be discussing at this point.