Start with this claim about causality:
Whenever there is something really common to many things, neither the multitude nor any member of that multitude suffice to explain it.
So if being human is common to Socrates and Joe, you can’t appeal to either Socrates or Joe to explain why there are humans. This is true even if one of them were the first human you happened to have, or if one was the father of the other. One needs an account like special creation or evolution or an alien plot, etc. If one decides on the alien plot, you might get an account of the human species, but not of embodied intelligences. The axiom in play is that the explanans has to differ in some way from the explanandum; this even will work as a replacement of the claim given above.
But then if all the objects we can experience have existence really in common, we’re committed to saying that any explanation of them has to be beyond existence. Interestingly, this gives us two things beyond existence: on the one hand the God of the philosophers who explains the totality of all that could be experienced or known by us, on the other hand the human mind that, in its own way, explains the totality as an object known to us.
Objection: Every explanandum requires some real thing that is different from it; but existence cannot admit of something real that is different from it.
Response: Being a whole or a totality is not repugnant to having a relation to another – in the cognitive order, for example, to understand the whole still requires some relation to a subject understanding, to desire to possess the whole requires some relation to the will, and to signify the whole still requires relation to its sign. If we deny that a whole can have relation to another, we can neither understand this whole or speak of it, which makes our very claim impossible to make. In the same way, being as a whole can still relate to another; and God himself can completely exhaust all the possibilities of being and existence in his pure actuality without ruling out the possibility of creation. Within the divinity itself, persons can be entirely whole and exhaustive of divinity while still existing in relation to other persons distinct from them.