If you follow this blog at all, you're probably the sort of person who knows what the "filioque problem" is. If you're not, go here.
My interest in the topic is great for two reasons: getting it right would both contribute to our knowledge of God and aid the cause of Orthodox-Catholic unity.
If you know anything about horsemanship, you know that the cliché 'beating a dead horse' does not refer to a paroxysm of futile rage but rather to an attempt to get somewhere with a vehicle unsuited to the purpose. To get a horse to gallop, one sometimes must give its flanks a whack or two; if the horse is dead, that method won't work. But this horse is not dead: theologians continue to whack its flanks intermittently. Given how old the battle lines are, however, success would not mean a gallop; a leisurely walk would do nicely, perhaps even better.
Our approach must be accordingly modest, avoiding all polemics; for as the late, great scholar of Christian doctrine Jaroslav Pelikan said: "If there is a special circle of the inferno described by Dante reserved for historians of theology, the principal homework assigned to that subdivision of hell for at least the first several eons of eternity may well be the thorough study of all the treatises--in Latin, Greek, Church Slavonic, and various modern languages--devoted to the inquiry: Does the Holy Spirit proceed from the Father only, as Eastern Christendom contends, or from both the Father and the Son (ex Patre Filioque),as the Latin Church teaches?" Save only, perhaps, for the topic of justification—a staple of Protestant-Catholic polemics—one is hard-pressed to find a theological topic on which more ink has been spilled to less constructive effect than the filioque. Beyond warning the unwary, the only justification for addressing it is to try to find a way to accommodate what each sides is, respectively, dogmatically committed to.
To that end, the first rule of discussion should be that any truth we can come up with on this topic will not add to our knowledge of God's inner life. There are far better means than academic theology for doing that—to the quite limited and indirect extent it can be done at all—and they are about living the Christian life to the full rather than disputation. What we can do by reasoning is to make more explicit and consistent what is already implicit in our dogmatic formulations of the deposit of faith. It's a matter of harmonizing our ways of talking, not of presuming, blasphemously, to do what is undoable. But even that modest goal would increase our knowledge of God by breaking down polemical battle lines and coming up with something that does justice to both sides' formulations. I've already set forth a bit about how; here I want to continue along that line.
In my brief earlier treatment, I argued that the filioque as defined by the Council of Lyons (1274), whereby the Holy Spirit proceeds from the Father and the Son "as from one principle" (quasi uno principio) can be logically derived from (a) what both sides profess in common and (b) two additional premises I take to be unexceptionable: God is essentially hypostatic, and the Spirit is the Spirit of the Father and the Son. Such a result is opposed by many Orthodox on the ground that it confuses "economy" with "theology" in such a way as to violate the mutually-agreed doctrine of the "monarchy of the Father," i.e. that the Father is the "sole cause" (monos aitia) of the existence of the other two divine Persons. Thus, while the Spirit proceeds from the Father and the Son in terms of the "economy," i.e. what God does for us, that does not and indeed could not follow in terms of "theology," i.e., what God is in himself. The way round that objection is to make some distinctions both sides could accept consistently with their dogmas.
If one takes the doctrine of the monarchy of the Father in a modest sense, to mean that the existence of the Father alone is underived, and that of the other two persons is derived from the Father's, there is no dispute. The dispute arises over the question whether the Son has something to do with the eternal "coming forth" (ekporeusis) of the Spirit's existence, a mode of derivation that is ineffably distinct from the Son's "generation" by the Father. The West has tended to take as axiomatic that economy reveals theology, so that the Spirit's "sending" of the Son in the "economy" reveals by analogy something about the Son's role in the Spirit's eternal procession in "theology," which is inchoately expressed by Lyons' definition of the filioque. So the question then becomes whether the result the West is thus committed to—i.e. the "theological" filioque—is compatible with the monarchy of the Father taken in a stronger sense, to mean that the Father alone is the cause of the being of the other two persons. People who take the monarchy in that sense are what Cardinal Dulles terms "strict monopatrists."
If strict monopatrism is true, then the theological filioque is false; for the former entails that the Son has nothing whatsoever to do with the Spirit's coming forth from the Father. But most Orthodox theologians seem, at the least, reluctant to say that. Thus a good many grant an "eternal manifestation" of the Spirit from both Father and Son, such that in that sense the Holy Spirit can be said to proceed from the Father "through" the Son. On that point too, there is no dispute; and if both sides had been suitably modest, the filioque problem could have been solved right there. For if the Holy Spirit proceeds from the Father through the Son, then there is a sense, perfectly compatible with Lyons as well as with the monarchy, in which the Holy Spirit proceeds from the Father and the Son. But the latter expression is vague, lending itself to other, arguably heterodox interpretations, such as the doctrine of the "dual procession" that seems to have been adopted by the Carolingians through their reading of Augustine. It has been agreed at least since Lyons that dual procession is out of bounds. So, in the final analysis, the dispute is really over whether the Spirit's eternal manifestation is somehow equivalent to his existence. If it is, then the theological filioque follows; if not, not.
The issue so framed hinges on the distinction between the divine "essence" and the divine "energies" from the Orthodox side ('DEE' for short) and the doctrine of divine simplicity on the Western side ('DDS' for short). Following the Palamite councils of the thirteenth and fourteenth centuries, most Orthodox insist that there is a "real" distinction between the essence and the energies, such that the Spirit's energetic manifestation through the Son does not logically entail the eternal procession of his being from the Son as well as the Father. Now if, by positing DEE that way, one urges that what-God-is does not necessitate what he does, there is a sense in which that is true and undisputed. What God has eternally decreed to do ad extra is in no way necessitated by what he is. On that the West should and does agree. But does it follow that what God eternally does ad intra is distinct in the same way from what he necessarily is? If so, then the theological filioque is false; if not, the question remains logically open.
Recall my earlier claim that God is "essentially hypostatic." That means that the Father's causing of the Son and the Spirit is necessary, not contingent: it's not as though the Father might have chosen not to generate the Son and breathe forth the Spirit; it is of the divine essence that he do so. On that, I take it, the Orthodox agree. The existence of the Son and the Spirit is accordingly logically equivalent to that of the Father: just as the Son and the Spirit exist only if the Father exists, so too does the Father exist only if the Son and the Spirit do. You get one of them if and only if you get all three. The question facing us at this stage of inquiry is similar: is the eternal but energetic manifestation of the Spirit ad intra logically equivalent to his existence? If DDS, the answer is yes and the theological filioque follows; if DEE, the answer would seem to be no, and the theological filioque consequently false.
I said the answer "would seem" to be no. Strict monopatrists have long taken that ball and run with it, which is why, e.g., Mark of Ephesus repudiated the reunion formula of Florence, a council where he was the chief Orthodox champion, and eventually induced the Orthodox world as a whole to do so. But things are not what they seem. As is so often the case, all depends on how terms are used.
If one takes the term 'divine essence' in DEE to mean what-God-is apart from what he does, then Mark's position follows almost trivially. What God does is, in certain respects, free, so that what he does is not necessitated by what he is; by the same token, what he is is not even communicated by what he does. That is why, in this kind of thinking, "theology" cannot be inferred from "economy." In that way, the essence of A is "really" distinct from A's manifestations and thus not logically equivalent to any of them. But DEE doesn't do the work that strict monopatrists require of it here.
For God is not just essentially hypostatic; God is essentially perichoretic too. The term perichoresis means 'mutually inhering' or 'mutually indwelling'; and as the Apostle John wrote, God is love. The love that is God ad intra consists precisely in how the Persons mutually inhere or indwell, which is something our minds can perceive only dimly and indirectly, by analogy, even as we are called to participate in it as children of God. That the Persons mutually indwell in love is, accordingly, not free: it is of the essence of God that they do so. What is free is only how they do so, since God is love and love, as distinct from desire or affection, is free. But if God is necessarily and thus essentially perichoretic in the way indicated, then we get the Spirit's eternal manifestation ad intra from the Father through the Son just in case we get the Spirit's existence, and vice-versa. The two are logically equivalent. From that, it follows that whatever holds necessarily of the Spirit's eternal manifestation ad intra holds of his eternal procession. It follows in turn that there is a theological filioque in a certain sense: it is the Spirit's eternal existence, not just his eternal manfestation, that proceeds from the Father through the Son, and therefore from the Father and the Son.
The filioque in that sense is not compatible with strict monopatrism, and thus is unacceptable to some Orthodox. Yet it's also a weaker filioque than the majority of Western theologians want and take for granted. But as far as I can tell, it's perfectly compatible with what each side is committed to dogmatically. Even if my way of framing it is not the best we can hope for, that sort of result is the best we can hope for. And I believe getting agreement on it is attainable.