I believe Robert Kovacs is representative in attributing to me the thesis of "symmetric interdependence" (SI) between the origination of the Son and that of the Holy Spirit. I take it that, by SI, Robert means the thesis that the origination of the Son by the Father depends on that of the Holy Spirit in the same way that the origination of the Spirit by the Father depends on the Son. I admit that my use of the term spirituque for the former would naturally cause people to attribute SI to me, and indeed the arguments against my position depend on that attribution. But I do not hold SI, and I need to make clear why not.
I do claim that the origination of the Son and the Holy Spirit respectively are "mutually interdependent." But from that claim, it does not follow that the interdependence relation is "symmetric," as though each depends on the other in exactly the same way, such that there is no order of precedence between them. All that follows is that it's impossible to have one without the other, not merely because both obtain co-eternally and necessarily, but also because they do so from the same source with whom they together constitute one substance, the divine substance. I see no need to argue on behalf of that claim for the benefit of Catholics. I call it the thesis of mutual interdependence (MI), which is a weaker claim than SI would be, for it does not rule out some-or-other order of precedence between the Son and the Holy Spirit even as it does not, by itself, specify any such order. Assuming there is some such order, the reason must lie in some further truth not specified by MI.
Robert's position is that the order of precedence ad intra must be specified as follows: the Son is a principle of the Spirit but the Spirit is not a principle of the Son. (That, I take it, is the position of Jonathan Prejean and Brandon Watson as well.) But it is not at all clear to me that said position is compatible with the monarchy of the Father. (I speak of "said position" because I want to avoid an exegetical debate about Aquinas' position, which Robert cites as an authority but whose unique aspects are not dogmatically binding.) Robert interprets the Lyons-Florence definition, i.e., that the Spirit proceeds from the Father and the Son "as from one principle," to mean that there is one common action, spiration, in which the Father participates as principle without principle and the Son participates as principle with a principle, i.e. the Father, from whom the Son has his being principle. But as it stands, that interpretation does not rule out double procession, an idea to which the Orthodox rightly object. Two hypostases, one of which is primary and the other derivative from the primary, are still two principles of the third, even if the action in which the first two participate is one common action, and even if the hypostasis that is the secondary principle derives its being principle from the hypostasis that is the primary principle. And two principles of procession constitute dual procession. Yet since the Lyons-Florence definition was deliberately adduced to rule out dual procession, and thus to address that very Orthodox objection, I firmly believe that we need to interpret "as from one principle" differently.
I interpret it thus: to say that the Spirit proceeds hypostatically from the Father and the Son "as from one principle" means that the Father originates the Spirit in virtue of being Father of the Son. That is the clearly orthodox sense of the filioque; it is entailed by, but does not itself entail, the objectionable sense. The former is thus weaker than the latter; but it is not just to say that the Father breathes forth the Spirit alongside begetting the Son, as if the former had nothing otherwise to with the latter. Rather, the Spirit is breathed forth by the Father only as the Spirit of the Father and the Son and is thus, as Paul VI confessed in his Credo of the People of God, as the bond or nexus of love between the Father and the Son. But the Son must not be said to contribute something to the act of spiration distinct from the Father's contribution; otherwise we would have dual hypostatic procession, which is ruled out ex hypothesi. Accordingly, while the Holy Spirit is consecutive to the Son inasmuch as the Son is, on my account, what I call an "explanatory factor" in there being a Holy Spirit, the Son is not said to be a cause of the Holy Spirit, even by derivation, in the way the Father is. That is also how I understand the traditional taxis by which the Son as hypostasis precedes the Holy Spirit as hypostasis, or the Holy Spirit is "consecutive" on the Son. I believe my interpretation to be clearly compatible with the monarchy of the Father, and thus acceptable to the Orthodox.
Now given such an interpretation, we may ask something similar about the begetting of the Son: can it reasonably be said that the Father begets the Son without the Holy Spirit's being in any way an explanatory factor in that act? Well, traditionally, Western theologians have argued that we must characterize the difference between the act constituting the origination of the Son, i.e., "generation" or begetting, and the act constituting the origination of the Holy Spirit, i.e. "spiration" or breathing forth, thus: the Son is involved in spiration but the Spirit is not involved in generation; generation is direct from the Father, having nothing to do with the Spirit; whereas spiration is with or through the Son. From that, it follows that the Spirit is not an explanatory factor in generation even though the Son is such a factor in spiration. But I don't think that will do. For if MI, then it is unreasonable to claim that generation takes place without being in any way dependent on spiration. Rather, I suggest, it is in virtue of breathing forth the Holy Spirit that the Father begets the Son, because it is impossible that there be a Son who is not related to the Father by a co-eternal and co-necessary Third, the Spirit, who is the bond of love between them. That, I take it, is what the PCPCU statement referred to in my previous post, which comes pretty much straight out of Gregory of Nazianzus, means. It must not be said, however, that the Holy Spirit contributes something to the act of begetting distinct from the Father's contribution. That would be dual generation, which is even more clearly unacceptable than dual procession. Rather, I claim, the Father and the Holy Spirit beget the Son "as from one principle," which means nothing other than that the Father begets the Son in virtue of his breathing forth the Spirit. That is also, I take it, what Weinandy means by saying that the Father begets the Son "in" the Holy Spirit. The Spirit is thus an explanatory factor in generation, even as the Son is an explanatory factor in spiration.
But the symmetry between generation and spiration is not precise. We may infer that the Father and the Holy Spirit beget the Son "as from one principle" only because, and as defined at II Lyons and Florence, it is revealed and professed that the Father and the Son breath forth the Holy Spirit "as from one principle." That epistemic asymmetry, I believe, reflects an ontic asymmetry by which a certain order of precedence must be said to obtain in reality, not just in thought. Thus the Son, the Second Person of the Trinity, is said to be the "Word" and hence the perfect self-expression of the Father, which is never said of the Third Person, the Holy Spirit, even though it must nonetheless be said that the Holy Spirit is co-equal in divinity to the other two persons. I suggest, perhaps unoriginally, that the Son "images" the Father immediately, whereas the Spirit only does so mediately, through the Son. By way of explaining the order of precedence, as weakly and inadequately as the subject matter makes inevitable, I invoke St. Maximus' analogy, whereby the Father is the "thought," the Son is the "word" embodying the thought, and the Spirit is the "breath" transmitting the word and thus the thought.
In human life, the kind of thought we're capable of is embodied in language, i.e. words expressing the thought, which in turn would not exist without physical tokens, i.e., speech and, later on in human development, markings. Hence the thought gives rise to the words and the tokens insofar as the latter two are mutually interdependent; but it is the words that more directly express the thought, giving meaning to the tokens. Hence the Son more directly expresses the Father than the Spirit, who can only be understood as the Spirit of the Father and the Son. But there would be no Son without the Spirit, just as there would be no words without speech.