From Tibbles to God
Tibbles exists inescapably asserts exists as a predicate.1 But as Peter Geach would tell us, it is an incomplete predicate, for it is meaningful only in relation – that is, dependent upon – the subject Tibbles.
Here’s the rub.
Tibbles, considered just in himself (in se), is an existential zero — hence his contingency. Tibbles existence cannot therefore depend upon Tibbles since Tibbles is nothing apart from his existence.
So if we admit that Tibbles existence is dependent (which we must) and that it cannot depend upon Tibbles, then Tibbles existence must be dependent on something other than Tibbles. We cannot logically maintain both that Tibble’s existence is dependent and that it is dependent on Tibbles. Tibbles by himself is nothing, so to be dependent on Tibbles would be to be dependent on nothing; but to be dependent on nothing is just not to be dependent at all. Thus, in saying Tibbles exists, we implicitly point beyond Tibbles to some extrinsic cause.
The move to God is straightforward from here. Take anything like Tibbles which, considered in se, does not exist – that must have existence imparted to it to be present in reality. All such entities are, by themselves, existential zeros. Can zeros be added to anything other than zero? Of course not. Finite or infinite in membership, if all reality were (collectively, existentially) zeros in se, nothing would exist. Because that is contrary to fact, there must one member in reality that is not really distinct from its existence – which is to say, whose what determination (essence) just is its whether determination (existence).
From there, the classical theist can deduce the divine attributes given the usual background metaphysics (existence and its convertibility with actuality, goodness, etc.).
PS - For more on cosmological reasoning, consider one of my (pay what you want) philosophy courses on Gumroad.
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For a rigorous defense of existence as a first-order predicate of concrete individuals, see Barry Miller’s The Fullness of Being.