Most of my vast readership knows that I've written fairly substantially about that issue before. (To find my articles thereon, just do an internal blog search on the phrase.) While I believe the doctrine is true as formally defined by the Catholic Church, I have at the same time made clear my belief that Rome's insertion of the phrase, a millennium ago, into the creed recited in the Latin Church was a needless blow to church unity. I have also made clear what ought to be uncontroversial: that rehashing old debates, whether about the phrase's insertion or the substance of the doctrine itself, is worse than useless. My interest in the topic is mostly to come up with a construal of it that is compatible with what both sides are dogmatically committed to affirming. And I am more convinced than ever that such is quite possible.
The longest thread ever seen on this blog was a combox on the latest article I've done on the issues dividing the Catholic and Orthodox churches. Unsurprisingly, over 50 of the comments were about the filioque. In his last comment therein, directed to me, the Orthodox theologian-in-training "Photios" Jones wrote (words between quotation marks are mine):
"As my earlier filioque article implied, to say that the Father breathes forth the Spirit "as" Father of the Son entails that the Spirit is caused by the Father..."
Okay. I think we've agreed here before. The Father causes the Spirit as Father of a Son.
"...Father of the Son entails that the Spirit is caused by the Father just in case the Father both generates..."
Let me ask a few question here: Are you implying that generation is a necessary condition for Spiration? I would also like to know if the reverse is true: is spiration a necessary condition of generation given that the Spirit is also "Spirit of the Father?" Or perhaps, you are only wishing to state the "condition" simply as the taxis (trinitarian order) relationship of the Persons?
"and has a perichoretic relationship with the Son.."
Same thing: Is perichoresis (of Father and Son) a necessary condition for Spiration? This is where I have the biggest problem because the perichoresis is absolutely in common between Father, Son, Holy Spirit. It would seem that for Them to have the same perichoretic nature, that perichoresis is dependent on spiration instead. Are you placing a middle [natural] attribute between Father-Son and Spirit as follows:Father - Son ---perichoresis--- Holy Spirit?
Given what I take to be the only legitimate purpose in discussing the filioque ecumenically, Photios' questions are fair. Herein I shall do my best to answer them.
The generation of the Son and the spiration of the Holy Spirit by the Father, which are the primordial instances of ekporeusis, are necessary conditions of each other. The Father not only breathes forth the Spirit as Father of the Son; he begets the Son as the one who breathes forth the Spirit. But even though we may thus say that the Holy Spirit "proceeds" (procedit) from the Father and the Son "as from one principle" (Lyons 1274: tamquam ex uno principio), we may not say that the Son is begotten by the Father and the Holy Spirit, as if it could be said that the Son is the Son of the Holy Spirit as well as the Father. We may and should say, however, that the Holy Spirit is the Spirit of the Father and the Son. This implies something about the order (taxis) of the Persons: the existence of the Spirit depends on that of the Son in a way in which the existence of the Son is not dependent on the Spirit. Although the existence of each is a necessary condition of that of the other, there's something more involved which would explain why the Holy Spirit is the Spirit of the Father and the Son but the Son is not the Son of the Father and the Holy Spirit.
That something, I submit, is the perichoresis of the Persons: their mutual indwelling in love. That the Spirit is Spirit of the Father and the Son does not mean merely that it is only as Father of the Son that the Father breathes forth the Spirit; it also means that a necessary condition of the breathing forth of the Spirit is that there be a perichoretic, and thus "energetic," relationship between the Father and the Son. Some Latin theologians have suggested that the Spirit just is that energetic relationship between Father and Son; that, at any rate, is what some have meant by following Augustine's dictum that the Spirit is the "bond of love" between the Father and the Son. The difficulty with that is not so much with what it says as with what it doesn't say: it does not afford clear grounds for presenting the Spirit as hypostasis, not merely energeia. Of course we can and must say that the perichoresis of all the Persons, like their existence, is eternal and thus unalterable, so that the perichoresis of all the Persons is logically equivalent to the existence of all the Persons. But that, in turn, doesn't capture the difference between the ekporeusis of the Spirit and the begetting of the Son. The Cappodocian Fathers were indeed most reluctant even to allow speculation about the difference. So, Houston, we still have a problem.
What I offer as a tentative pointer to a solution is this: the difference in taxis I've described is accounted for by whatever-it-is that accounts for why the Spirit is Spirit of the Father and the Son but the Son is not the Son of the Father and the Holy Spirit. I do not profess to know the difference; and I suspect that, even if somebody knew it mystically or by some special revelation, they would not know how to state it and would find it almost blasphemous to try. Perhaps a metaphor of St. Maximus' would help: the Father speaks his thought by begetting the Word and breathing forth the Holy Spirit. The "breath" of God's self-expression thus has a certain order in which the Son comes before the Spirit, even though the former does not and cannot exist without the latter.