The book I quoted from previously gives three forms of the PSR:
Weak: Whatever begins to exist has a cause for its existence.
**Mild: **Whatever exists contingently has a cause for its existence.
Strong: Whatever exists has a reason for its existence, either in the necessity of its own nature or in the casual efficacy of some other being.
Only the strong form of the PSR presented here would include God in its scope, and we are not at this time thinking of the “God of Abraham”, but of the Prime Mover and First Cause, that is, that which has been logically deduced to be necessary in itself; the terminus of any essentially ordered series of movers and causes, in whom there is no composition, not even of essence and “act of being.” God would therefore fall into the part of the PSR in which the being has the reason intrinsically, or “God’s reason for existence is in the necessity of His own nature.” God doesn’t just have an “act of being,” He *is *the “act of being.” Since what He is is the same as that He is, He requires no cause to explain a composition of an essence with an “act of being”, there is no composition, He simply is it and so possesses it intrinsically.
You might object with “why can’t this teapot just be said to have an act of being intrinsically,” and I think that would imply that attributing to God an intrinsically necessary “act of being” is simply an arbitrary choice on the theist’s part, but the point is that the teapot example is clearly false, as would anything else that is a composite of parts, substance and accident, form [Aristotlean] and matter, essence and esse.
The cosmological arguments don’t so much explain why God has the act of being intrinsically, it’s (1) that anything that has properties X, Y, and Z cannot be the Prime Mover or First Cause, (2) that there must be a Prime Mover and First cause (which we will denote as God; #2 is not being demonstrated here, that’s the point of the cosmological arguments which I’m not recapping), and (3) if 1 and 2 are true this God must be divinely simple, actus purus, a being who simply is His own act of being. The cosmological arguments propose that there must be a being who is his own act of being by intrinsic necessity, and this is what we call God. To see it as something simply being arbitrarily attributed would be to miss what is established by the arguments. It’s not explaining “why” exactly so much as saying “there must be something that has it intrinsically if the PSR is true,” because if there isn’t, everything is ultimately rendered a PSR-defiant being."
