A posteriori arguments

The ontological argument provides, as we have seen, an opening onto many important metaphysical questions about possibility, necessity, and existence.

there can be no infinite regress in the production of things from their materi­als, as flesh from earth, earth from air, air from fire, and so ad infinitum. Nor in the agencies whereby changes are effected, as a man is moved by air, air by the sun, the sun by strife, and so ad infinitum.

Aquinas says that it follows from the impossibility of an infinite regress that there must be a primum movens immobile (a first mover that is itself not moved) and that that is God. The conception of God here, then, is as the prime mover. This is the first of the famous arguments. In a parallel way, he argues that every sequence of cause and effect must have a beginning, and so there must be a first cause that is itself not caused. That is the second argument, often called the “cosmological argument.”

The third argument is that the existence of contingent beings— beings that might not have existed—implies the existence of a nec­essary being. And the fourth is that there must be some absolute standard with which to compare the relatively imperfect beings that make up the created universe.

These four arguments share one difficulty with the ontological argument: if successful, they establish the existence only of a very abstract entity, something rather different from the God conceived of by most believers. We should also ask whether the prime mover and the first cause, the necessary being, and the standard of perfection are the same person (or thing). And that might prompt us to wonder fur­ther whether he (she? it?) is the same as the being that many believ­ers have some conception of. We have, that is, with these arguments the possibility of confusing different things that happen to have the same name, as happened with Michael Jordan and Michael Jackson. In addition to these proofs, then, we should like to have some argu­ment that they establish the existence of the same person (or thing).

These arguments are, however, also unlike the ontological argu­ment in an important way: they are a posteriori. They begin with premises about the actual world: that there are things that move one another, causal chains, contingent and imperfect beings. It is start­ing from these facts that we proceed to the conclusion that God exists: God conceived of as prime mover, first cause, necessary ground of contingent being, and perfect standard. Aquinas saw this as essential to any valid argument for the existence of God, because he thought all valid arguments would have to argue from God's effects to his existence as their cause.

Aquinas's first four arguments have not had many philosophical defenders recently. The first two arguments seem to suppose some­thing like the principle of sufficient reason, which I mentioned at the end of Chapter 4—the thesis that every event has a cause. Given the fact that modern physics appears to proceed without the princi­ple of sufficient reason, we have reason to doubt that it is an a pri­ori truth; we have reason to doubt that it is a truth at all. (They also rely on the controversial assumption that there cannot be an infinite series.) The third argument has all the problems with necessary exis­tence that we saw in dealing with the ontological argument. And the fourth just does not seem very convincing.

But Aquinas's fifth argument lives on, and it is not only the sub­ject of lively philosophical discussion, but also the foundation for the religious beliefs of many people now and throughout human history. It is called the “teleological argument” or “the argument from design.”

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