Oooh, this is an intriguing question
@SalixIncendium
In Nicene Christianity, we obviously have the doctrine of "
one ousia [essence] in three hypostases [usually rendered 'persons' but more properly 'subsistent relations']". In the creed, the Lord Jesus is referred to as "
God from God, Light from Light, true God from true God, begotten, not made, consubstantial with the Father".
St. Thomas Aquinas’s tells us in the
Summa that the “
the act of God’s intellect is His substance (essence)” and thus His self-consciousness as an object in Himself is common to the Persons as one 'being', rather than individuated severally. However, the Christian revelation equally informs us - and over-above 'simple monotheism' - in a special way that this Divine Being does not exist in eternal 'solitude' but rather is 'supremely happy in Himself' with Himself.
The traditional scholastic formula (from St. Thomas Aquinas) thus contends that: "
God is not three consciousnesses but One Consciousness who subsists in a threefold relation of Father, Son and Holy Spirit", to quote the great divine Robert South (1634 – 1716) in his
Animadversions against the tritheistic heresy of William Sherlocke (who, inspired by Descartes, preached a "
unity of shared consciousness between three infinite divine minds").
What's interesting is that the Father generates (and is 'unoriginated' Himself), the Son is begotten of the Father and the Holy Spirit proceeds from both. God is perfectly one in the 'absolute' sense, except for these eternal relations of origin in the 'relative' sense. So there is, within the 'inner life' of God as He relates to Himself, three 'relations' of origin, whereby we can understand the 'Father' to be the source of the Godhead.
However, the Persons are co-equally the one "
summa res" (Supreme Reality) in essence, being, will, intellect and consciousness - so that in Himself, God is one undivided and ineffable Being (not three distinct 'conciousnesses' or agents, which would amount to three Gods) - even though they are really distinct from each other relatively speaking).
St. Augustine of Hippo provided our western theological tradition with its classic expression of Trinitarianism, through a
psychological analogy of the
Trinity in which the unity of
essence is compared with the rational part of the human soul, composed as it is of “
the mind, and the knowledge by which it knows itself, and the love by which it loves itself.” The Son's 'generation' from the Father is not from 'cause to effect' (which would make Him an inferior, dependant entity to the Father) but rather is an 'intellective process' whereby the one Mind of God comes first to know or contemplate Himself and generates His own (mental) self-image (hence why we call Jesus the 'Word of God') which is the Son of God. The love with which God loves or delights in Himself is the Holy Spirit, the 'bond' of love between the Father and the Son.
As St. Augustine of Hippo explained:
CHURCH FATHERS: Confessions, Book XIII (St. Augustine)
But the three things I speak of are, To Be, to Know, and to Will. For I Am, and I Know, and I Will; I Am Knowing and Willing; and I Know myself to Be and to Will; and I Will to Be and to Know. In these three, therefore, let him who can see how inseparable a life there is — even one life, one mind, and one essence; finally, how inseparable is the distinction, and yet a distinction. Surely a man has it before him; let him look into himself, and see, and tell me.
But when he discovers and can say anything of these, let him not then think that he has discovered that which is above these Unchangeable, which Is unchangeably, and Knows unchangeably, and Wills unchangeably. And whether on account of these three there is also, where they are, a Trinity; or whether these three be in Each, so that the three belong to Each; or whether both ways at once, wondrously, simply, and vet diversely, in Itself a limit unto Itself, yet illimitable; whereby It is, and is known unto Itself, and suffices to Itself, unchangeably the Self-same, by the abundant magnitude of its Unity — who can readily conceive? Who in any wise express it? Who in any way rashly pronounce thereon?
So, I would have to make a distinction here beween "
God as he is in Himself" (absolutely) and
"God as He relates to Himself" (relatively). In the first sense considered absolutely in Himself, there is no 'God from God': just one supreme, inexpressible divine reality which neither begets, is begotten nor spirates but just '
Is' and knows, wills, loves etc. In the second sense, however, God has knowledge of Himself by generating a distinct Image of Himself and loves Himself through that contemplation, such that there is a kind of 'internal' hierarchy of 'relations' (distinct manners of subsisting of the one divine essence).
This doctrine of God was summarised, as follows, by the Fourth Lateran Council (1215) [which for Catholics is the twelfth ecumenical council]:
Fourth Lateran Council : 1215 Council Fathers - Papal Encyclicals
We, however, with the approval of this sacred and universal council, believe and confess that there exists a certain Supreme Reality [summa res], incomprehensible and ineffable, which truly is the Father and the Son and the holy Spirit, the three persons together and each one of them separately.
Therefore in God there is only a Trinity, not a quaternity, since each of the three persons is that reality — that is to say substance, essence or divine nature-which alone is the principle of all things, besides which no other principle can be found. This reality neither begets nor is begotten nor proceeds; the Father begets, the Son is begotten and the holy Spirit proceeds. Thus there is a distinction of persons [subsistent relations] but a unity of nature [essence].
Although therefore the Father is one person, the Son another person and the holy Spirit another person, they are not different realities, but rather that which is the Father is the Son and the holy Spirit, altogether the same; thus according to the orthodox and catholic faith they are believed to be consubstantial. For the Father, in begetting the Son from eternity, gave him his substance, as he himself testifies : What the Father gave me is greater than all. It cannot be said that the Father gave him part of his substance and kept part for himself since the Father’s substance is indivisible, inasmuch as it is altogether simple.
Nor can it be said that the Father transferred his substance to the Son, in the act of begetting, as if he gave it to the Son in such a way that he did not retain it for himself; for otherwise he would have ceased to be substance. It is therefore clear that in being begotten the Son received the Father’s substance without it being diminished in any way, and thus the Father and the Son have the same substance. Thus the Father and the Son and also the holy Spirit proceeding from both are the same reality.
