What is not individual is common
William B. of Neochalcedonian reprises a mainstay of the anti-Western argument on the Envoy forum. Coming as this reply does following the plea for charity from Mike Liccione and Brandon Watson regarding this very same topic, I am hoping that it will be taken in a good spirit, despite being a mite critical of one of one of the great Eastern Fathers.
William reproduces an argument from a soon-to-be-ex-Byzantine Catholic (I'm guessing Todd Kaster) that could have come straight from St. Basil the Great:
Spiriation is either a personal attribute (specific to one Person of the Holy Trinity), or an attribute common to all three Persons. If both the Father and the Son spirate the Spirit, then the Spirit must spirate as well, and then there would be endless spirations and endless Persons of the Trinity. You can't say that two Persons spirate, but the third Person doesn't
It's the same argument advanced by St. Basil, and alas, it relies on the same false dichotomy that his did. Ever wonder why St. Athanasius wasn't swayed by St. Basil's plea to condemn Marcellus ("do not despise the hypostases")? I think it's because St. Athanasius knew St. Basil's formulation was inessential, so that it wasn't a good basis for condemning anyone. That's not to say that Athanasius would have followed Marcellus in equating the manifestation of a reality with the reality itself (indeed, it is fair to say that he certainly would not have), but he appeared to have considered Basil's requirements insufficient to justify the condemnation.
So why doesn't it follow from homoousion that whatever is not individual is common? This idea that it does results from the premise that individuals are realizations of natures, which in turn results from the equation of being with form (and non-being with formlessness). Thus, for example, the eidos of an individual is the combination of body and soul, which together form a single logos. There is not composition of existence and essence, but rather, there are commonalities between many logoi (natures) and individuality that distinguishes them. Activity is explained in that each nature (logos) has a characteristic activity (dynamis) that is exercised according to an individual mode of use (tropos, see also divine persons as tropos hyparxeos = "mode of existence"), viz., the possibility of activity is referred to nature while the actuality is referred to person. In general, and particularly with regard to deification, the entire distinction between created and uncreated becomes the One-Many problem writ large: how does the Creator unite all things in Himself so that the existence of multiple things does not disrupt his unity?
St. Maximus deployed various aspects of Neoplatonism in offering a solution of this problem, rebutting the belief that "distinction is opposition" and the notion that distinction inevitably required fragmentation. His Christological and Triadological solutions were applications of this method, and it appears to be that, pace Eric Perl, he did not derive his solution from neo-Chalcedonian Christology. Rather, like his predecessors (including the famous Leontioi), he deployed Neoplatonic concepts in his explanation. See Melchisedec Toronen, Union and Distinction in the Thought of St. Maximus the Confessor.
That more or less suffices for the East, but why was the West different? I think that it starts with having a Stoic (Tertullian) as a magisterial figure in Triadology. The reason I think this is the case is that it introduced room for a tertium quid. Remember that in the Stoic picture, there is an immanent energy (fire, reason, providence, or most aptly, pneuma) that shapes matter, so there is a feeling that everything is the same "stuff" (matter) except as to the extent that it is active in some particular. Essentially, a thing is its doing; what it does constitutes what it is. Tertullian is extremely materialistic in his understanding of this idea; he thinks of the immanent principle as immanent "stuff" (which he calls "spiritus") that is the principle of material activity. If you look at what he says in Against Praxeas 2-3, it becomes pretty clear that he thinks of the Son and the Spirit as having been given this power in their entirety, despite being separate in operation, while lesser spiritual beings (angels) receive this power to a lesser degree, with humans (as material but ensouled) trailing just behind that. Thus, the Father, Son, and Holy Spirit differ in degree, form, and aspect (gradus, forma, species), but not in condition, substance or power (status, substantia, potestas). That turns out to be the key point in the Homoian controversy; non-identical activities (viz., different acts of manifestation) can show the same power.
What this has done, although it isn't apparent except by comparison, is that it has introduced conceptual space (a diastema, even) between the individual form and outward (non-exhaustive) activity. The significance of that gap has been grossly underestimated; as far as I can tell, it is cataclysmic. Recall: the Eastern view localizes the difference between essence and activity in the gap between what would in Aristotelian parlance be first substance (individuality) and second substance (essence). That's why it is so crucial to locate the source of personal existence and personal activity in an individual and to separate what is common from what is individual. In the Western view, on the other hand, the only necessary distinction was exhaustive/non-exhaustive. This bears no coincidental resemblance to the relationship between infinity and finitude, with the Trinitarian life being actually infinite and the finite forms of communication being divisible into the truly finite and the potentially infinite (viz. deification; see also Clement of Alexandria on Christ's "magnitude"). The fact that each Person could truly reveal the divine in different ways (and indeed, the divine power entailing the freedom to do so, as Dr. Liccione points out) opened up the possibility that the terminus of an internally exhaustive transfer could manifest itself in the Economy in entirely different ways (which acts of manifestation Marcellus unfortunately confuses with the persons themselves). And the concept of matter/potency as a real delimitation of the composite provides an excellent metaphysical complement to the distinction between the infinite and finite, between exhaustive act (actus purus) and created acts.
The Incarnational question was then to understand how the same subject brought these two modes of action into one reality, which led to two-form Christology (Phil. 2:5-11). The Triadological question was to explain how, in light of the affirmation of the Son and the Spirit (which existence would otherwise be opaque from the activities alone), that the intra-Trinitarian relationships were exhaustive. Augustine came up with his famous "psychological analogy," which oughtn't be taken too literally, as it is only intended to show the conceivability of a relationship that is exhaustive in terms of creating identity. Anselm's argument can quickly be used to demonstrate that, if there is more than one Person, there can be exactly and only three based on the bipolarity of the relations, and Photius's reductio doesn't work on it (the relations are exhaustive, ergo non-separating by the infinite/finite distinction). That distinction was adopted quite comfortably in the West, providing a rather convenient answer to the Homoian Arians as to how different economic activities of the persons did not show different natures and to how begottenness/spiration does not imply subordination. Consequently, it ain't exactly surprising that the author of the Tome to Flavian has no qualms about using the filioque in defending orthodox Triadology. While it's understandable that this might have looked a bit curious even to sympathetic Greeks (like St. Cyril), it seems to me that it is a self-consistent explanation of the data of revelation, even if it departs in some respects from points of Cappadocian theology. To really see the underlying contrast up close and personal, it is useful to compare the difference between St. Augustine and St. Gregory Nazianzen on the subject of schesis or to compare Nicholas of Cusa's view of the Neoplatonic infinite sphere (building on Eckhart's description of God as a sphere whose "center is everywhere and radius is nowhere") with St. Maximus's account, as recounted by Toronen (see above). Cusanus is clearly dealing with the act of existing (as distinct from essence) while St. Maximus is dealing with the one/many problem of logoi.
I don't say this to trash St. Basil or St. Maximus relative to St. Thomas or Cusanus. I confess that I find the Western account makes more intuitive sense to me, because I've been immersed in Western physical science practically from birth, meaning that Aristotelianism is ingrained at this point. But the Western account is vulnerable to misreads at the point of divine causation, so if you're looking for a clear affirmation of libertarian free will (which is necessarily mysterious in the Western account, meaning that you have to punt on how God and humans are free), Maximus is probably better. The Western view is more abstract regarding sin but more concrete regarding the centrality of the Lord's Passion. But the overarching point I would make is that they aren't in conflict, unless they are forced to be.
Brandon Watson gives some sage advice in this regard about the issue at stake in simplicity being wholly different between Thomas and Palamas. There have been enough attempts to try to beat the other side into an incompatible metaphysical framework in order to cast the opponent as a long-despised enemy. This problem is not going to go away until we realize that some ideas aren't portable between the sides but that this is not a good basis for charging the other side with hopeless inconsistency.