Thomas Aquinas explained the Triune Persons as subsistent relations: “As to essence, the Father is in the Son because the Father is his essence and he shares it with the Son without any change taking place in himself” (ST I, 42).
Stephen Long explains Thomas’s claim that Father, Son and Spirit are “subsistent relations” this way: “It means that God is not like us, for we are not constituted entirely by our relations. No matter how much I seek to give myself to my wife, my children, my friends, I cannot give myself to them in the same way that the Triune Persons give themselves to each other. Nor do I exist because of that reciprocal giving and receiving. No matter how hard I try, I am not a ‘subsistent relation’ – because I am not God.”
Long’s point here is to insist that “God is Not Nice,” that is, God does not exist to make us happy, to be a means for our spiritual fulfillment. He concludes that “God can do more than partially give God’s self. That is why we say the Triune God is defined by subsistent relations. And what does that mean ‘for us’? Not much, thanks be to God. It means that God does not need us for the sake of God’s own self.”
I agree with Long’s conclusions, but have reservations about the path of the argument. First, as to Thomas: In the quotation, Thomas is explaining the interpenetration of the Persons in terms of the interpenetration of the essence of God in each Person. The Father is His essence; therefore, when He shares His essence with the Son He is also sharing Himself. It is not possible to say that the Father bequeaths, as it were, His essence to the Son without offering Himself as Father, since His being is identical to His being Father.
It appears that this problem can be avoided only by insisting that the Persons are not subsistent relations.
Second, as to Long: He locates a discontinuity where I want to stress continuity, or at least analogy. He describes being a subsistent relation in terms of the degree to which one can give onself away. The Triune Persons are subsistent relations because they can give themselves to each other without remainder. The Father wholly gives Himself to the Son, and so on.
But, again, if this is true, then does the Father give His quality of Fatherhood to the Son? If not, then it appears that there is something “reserved” to the Father even when He gives Himself to His Son, and the “reservation” is precisely what makes Him Father and not Son, and prevents the Son from becoming a second Father. But then there’s a much closer analogy to human existence: Whenever we give ourselves to another, there is always some “reserve” that is not given; though my self-offering may well make the other much like me, that reserve is precisely what maintains my integrity in distinction from the other, and vice versa.
Without that reserve, it is hard to see how we can speak in terms of self-giving at all; there would seem to be no there there to give or receive. There is certainly a discontinuity here between Triune relations and human personal relations. The Father gives Himself in the Spirit to the Son in a way infinitely beyond my self-giving to my son.
Yet, it remains that the Father who is in the Son is in the Son as Father. It would be wrong to call this a “partial” self-giving, but it is not absorption either.