Thank you for your explanation. I understand there is a “principle of triadology” employed by the Eastern Orthodox (or at least by their apologists) which states that any property of the Divine Persons must be shared by all or held only by one. Like you say later that Eastern Orthodox do not accept the principle that the persons of the Trinity must be distinguished by relations of opposition, it is not clear to me why the EO principle must be assumed. Is there a philosophical or scriptural basis for it? Where did this principle develop historically?
It is a direct consequence of the Cappadocian distinction between hypostasis and ousia. Essentially, it is a response to the challenge of Eunomianism. For Eunomius, the Father’s being unbegotten is His very essence (and all of His attributes as a result must be identical with this attribute). For the Cappadocians, the three Trinitarian Hypostases are distinguished by their manner of existence (τρόπος ὑπάρξεως), which is identical with their hypostatic characteristics. To share in the hypostatic characteristics of another, because the hypostatic characteristics are identical with the τρόπος ὑπάρξεως of that hypostasis, would be a blending of hypostases, a participation in the very manner of that hypostasis’ existence.
For us, if Causality is a property of the divine nature, then the Holy Spirit too must be its own cause; and if Causality is an hypostatic characteristic of the Father, then the Son cannot share in this without having His manner of existence confused with the Father’s manner of existence; and if the Father and the Son are said to share in Causality without a blending of hypostases, and without the Holy Spirit being is its own cause, then this implies that the Father and the Son share properties with each other in a more intimate way than they do with the Holy Spirit (in other words, implying that there is some substance in common between the Father and the Son in which the Holy Spirit does not share), which is quite subordinationist (not to mention the ramifications such a consequence would have for the doctrine of divine simplicity).
Secondly, even if this principle is admitted, I don’t think it must necessarily contradict the Filioque because even though the Father and Son are a single principle of the Holy Spirit, the sense in which the Son participates in the spiration of the Holy Spirit is not in every respect exactly the same as the sense in which the Father spirates the Holy Spirit.
I don’t disagree that the filioque is not necessarily contradicted by this principle, because as St. Maximus teaches in his letter to Marinus, there is an Orthodox sense of the filioque, in which the Son is not made into the cause of the Holy Spirit, but rather the Father alone is the cause of the Son and the Holy Spirit (one by Begetting and the other by Procession). According to St. Maximus, the Latins of his day used the expression filioque in order to teach the progression (προϊέναι) of the Spirit through the Son. The question then is what do the modern day Latins who affirm the teaching of Florence mean when they confess the Filioque? Do they not mean, in accordance with the teachings of the council of Florence, to confess the Son as principle and cause of the subsistent being of the Holy Spirit? If so, then we cannot accept that, because we find it to be foreign to the teachings of the Holy Fathers, most especially of St. John of Damascus, St. Gregory the Theologian, and St. Maximus the Confessor.
Third, if this principle is applied so strictly, doesn’t it undermine the EO doctrine as well? The Holy Spirit is subordinated by the Latins because they say that Son and the Father share in causality to the exclusion of the Holy Spirit (poor guy). However, the EO say that the Son and the Holy Spirit share in being caused, leaving the Father out. Do the EO subordinate the Father by saying this?
Properly speaking, being caused is not said univocally to be a property of the Son and the Holy Spirit, but rather being begotten and proceeding are their hypostatic properties. The two, according to St. Gregory the Theologian and St. John of Damascus, differ in an unknown way. Indeed, if it were that the Son and the Holy Spirit were both properly said to be caused in a univocal fashion, then there would be no difference between the two, and the Holy Spirit would be another Son, and the Son would have a brother. Furthermore, even us creatures, if ‘being caused’ were applied univocally to us, would be brothers of the Son, and Sons of the Father. We do not run into a problem, however, because we understand that ‘being caused’ is not used univocally.