The God of the Third Way is what is necessary by itself. The Third Way explicitly recognizes the possibility of necessary beings other than God, but it only insists that they are necessary by another. But we obviously can’t claim in the midst of a proof for God’s existence that the universe or matter are necessary by another because God made them; and if we have some criterion that establishes that the universe is necessary by another, then why don’t we just apply that criterion and dispense with the Third Way? How do we keep the Third Way from either begging the question or being superfluous?
One response is that St. Thomas is working from a hypothetical account of the what is necessary by itself, perhaps like
If a thing is necessary in itself, then knowing its definition would suffice to make us know it exists.
To justify: one sense of “in itself” is “by definition”, and the necessary (in the context of the Third Way) is what exists at all times.
It’s pretty clear that no physical thing could be necessary in itself: a cogito argument could make us know that a certain physical individual exists but individuals are not defined, and any physically defined entity requires information beyond the definition to make its existence given. When we run any necessary thing other than God through this, all of them fall short of being necessary in themselves, but we still need the Third Way to establish that there is at least one thing that is not just necessary, but necessary in itself.
The Ontological Argument is close at hand in all of this. STA seems to think that the conditional is true but that we don’t in fact know the definition of what is necessary in itself. This requires some account of what we’re doing when we isolate essentially true and convertible accounts of God as unmoved mover, agent cause without agent cause, necessary being in itself, etc. but part of the answer would consist in the fact that individuals are indefinable.