In my view the definition of truth is this: not to have a mistaken apprehension of Being. Falsehood is a kind of impression which arises in the understanding about nonbeing: as though what does not exist does, in fact, exist. But truth is the sure apprehension of real Being.
. . .
It seems to me that at the time the great Moses was instructed in the theophany he came to know that none of those things which are apprehended by sense perception and contemplated by the understanding really subsists, but that the transcendent essence and cause of the universe, on which everything depends, alone subsists.
For even if the understanding looks upon any other existing things, reason observes in absolutely none of them the self-sufficiency by which they could exist without participating in true Being. On the other hand, that which is always the same, neither increasing nor diminishing, immutable to all change whether to better or to worse (for it is far removed from the inferior and it has no superior), standing in need of nothing else, alone desirable, participated in by all but not lessened by their participation—this is truly real Being. And the apprehension of it is the knowledge of truth.[1]
The argument from contingency is, in my estimation, probably the best demonstration of the existence of God, and in fact, the necessary existence of God. Gregory begins with the idea of truth being the apprehension of what is, of being. This is because Gregory is discussing knowledge here, not the existence of God, but the two are intimately related, since the existence of God can only be apprehended if we understand how we can know something is true or not, and to know that, we need to understand the nature of truth. Gregory of Nyssa understands Truth to be the apprehension of real being, as opposed to nonbeing. This understanding will have certain implications, for example it would lean towards one being a Platonist when it comes to universals, mathematical objects, etc.
The next two statements get to the heart of the issue: nothing in the perceptible world subsists in itself, in other words, everything in the world is contingent on something else for its existence. This, Gregory states, is understood simply with reason, I agree. Everything that exists materially could not exist; in fact, no matter could exist at all, or energy, there could be no material universe. Every instance of being in the perceptible world exists by something else: a tree exists because there is soil for it to grow, a star exists because of the gasses that make it up and the laws of nature that hold it together, electrons exist as part of the framework of fundamental particles and the energy and forces that order them. The contingency is both downwards (things existing because of their parts) and upwards (things existing due to higher laws and frameworks), it is also temporal and horizontal, a river is caused by rain, black holes are caused by dying stars, atoms are caused by the forces of attraction, etc. etc. all the way back to the big bang.
Ultimately, however, all these contingencies that are interconnected have an ultimate contingency: that of being itself. All that is, IS, it exists, what is it to exist? Obviously, that is the question that has plagued philosophy from the pre-Socratics through Plato, Hegel, Kant and Heidegger; needless to say, it’s a question that continues to be asked. Gregory’s answer (along with much of the Classical Theist tradition, as well as the neo-Platonic tradition) is that to be is to participate in Being: the metaphysics of participation. This ties in with the idea of mutability, potentiality and actuality. To give an example, a seed exists as a seed, but it has the potential to become a tree, when it is planted, it “becomes” a tree, and thus “is” a tree, through its becoming a tree, as a tree it participates in being, it IS, as a tree, but also with the potentiality to become firewood, or be burnt in a forest fire and become ash. It is therefore always becoming, participating in being as what it is, from a potential of what it was, and with potentials of what it could be. Ultimately, all those things which exist as certain things, with potentialities, and finitude (existing as this or that) depend on what Gregory calls “true Being”: God. God exists as true being, real being (as Aquinas would put it, his existence is his essence) he doesn’t exist finitely or contingently, but simple IS. It is this IS that all modalities of finite being participate in in their beings as certain things, with potentialities, they all participate as finite instances of infinite being.
The other option is that being just is, in its instantiations, that being doesn’t participate in an ultimate reality (God), but just is as it is in itself. The problem with this is that all contingent being exists as contingent, mutable, with potentialities and possibilities (including the possibility of non-being), none of those possibilities, contingencies, etc. exist in the instantiation, they are all dependent on outside, and in fact, all contingent being itself cannot ground itself because all of contingent being itself could have not been, it itself is an instantiation of being that could have been something else or nothing at all, and it in it self does not hold the explanation for itself.
Ultimately, Gregory of Nyssa’s maxim is true: reason demonstrates that nothing exists self-sufficiency, but all exist by participating in true Being itself. Not only that, but since truth is that which coincides with what is, ultimate Truth is knowledge of true Being (Psalms 145:3; Acts 17:27). Understanding that all finite being exists as a participation in true Being, in self-subsisting Being, in God, is not only inevitably true, but a vital starting point of any Theology proper.
[1] Gregory of Nyssa, Life of Moses, 2.23–25.
Musing on Theology 