Gene Callahan linked to a very interesting post by Ed Feser. It begins in this way, and note that I’m going to abandon my normal formatting style and keep the italics the way Feser wrote it:
[FESER:] For the Thomist, to say that God is the First Cause of things is, first and foremost, to say that He is the cause of their existence at every moment at which they do exist. God creates things out of nothing precisely in the act of conserving them in being, and apart from His continual causal action they would instantly be annihilated. You, the computer you are using right now, the floor under your feet, the coffee cup in your hand – for each and every one of these things, God is, you might say, “keeping it real” at every instant. Nor is this causal activity something anything else could either carry out or even play a role in. Creation – which for Aquinas means creation out of nothing – can be the act of God alone.Where creation is concerned, then, God is “first” cause not in the sense of coming before the second, third, and fourth causes, but rather in the sense of being absolutely fundamental, that apart from which nothing could cause (because nothing could exist) at all. As serious students of the Five Ways know, the sorts of causal series Aquinas traces to God as First Cause are causal series ordered per se, not causal series ordered per accidens. In the former sort of series, every cause other than the first is instrumental, its causal power derived from the first. (See this post for more on the subject.) But where creation is concerned, Aquinas’s talk of intermediate or instrumental causes is only “for the sake of argument”; his point is that even if there were intermediate causes of the being of things, the series would have to terminate in a First Cause. In fact there is and can be only one Creator and He cannot in principle create through intermediaries. (That is not to say that God does not work through intermediaries in other respects. We’re only talking here about His act of causing the sheer existence of a thing or creating it out of nothing.)
Why not? Aquinas addresses the question at some length in the Summa Theologiae, the Summa Contra Gentiles, and De Potentia Dei. The arguments are difficult for someone not versed in the metaphysical presuppositions of Aquinas’s philosophical theology – indeed, some of them are difficult even for someone who is versed in the relevant metaphysics.