The First Cause Argument

St. Thomas Aquinas (1225-1274), whose writings on theological matters are regarded as authoritative by the Catholic Church, is the author of this famous argument (although he borrowed heavily from Aristotle's "unmoved mover" argument). The basic argument is very simple: everything in the world around us has a cause; but the chain of causation cannot extend back into infinity; therefore, there must be a first cause, and this is God. We may call this the popular version of the argument. Aquinas's full argument is somewhat more complex, however. Here is the original version from Aquinas's "Five Ways", as translated by Ronald Rubin:

"In the world that we sense, we find that efficient causes come in series. We do not, and cannot, find that something is its own efficient cause — for, if something were its own efficient cause, it would be prior to itself, which is impossible. But the series of efficient causes cannot possibly go back to infinity. In all such series of causes, a first thing causes one or more intermediaries, and the intermediaries cause the last thing; when a cause is taken out of this series, so is its effect. Therefore, if there were no first efficient cause, there would be no last or intermediary efficient causes. If the series of efficient causes went back to infinity, however, there would be no first efficient cause and, hence, no last or intermediary causes. But there obviously are such causes. We must therefore posit a first efficient cause, which everyone understands to be God."

(Note: Aquinas uses the term "efficient cause" because in Aristotelian metaphysics there are four different types of cause. But an efficient cause is roughly equivalent to what we now mean simply by "cause", so the term "efficient" may be ignored.)

There are two interesting parts to this argument that are left out of the popular version. First, Aquinas argues that nothing can be its own cause. For, in order for something to be a cause of anything, it must already exist. Thus, if something were to be its own cause, it would have to exist before it exists — a clear contradiction. His conclusion, therefore, is not that God is self-caused, but rather that God is uncaused.

One may question this on the grounds that perhaps there can be instantaneous causation. If there is no time between a cause and its effect, then a self-caused entity would not have to exist before it causes itself to exist. Aquinas's conclusion therefore does not follow unless one rules out instantaneous causation.

In the second part of the argument, Aquinas attempts to show that the chain of causation must have had a beginning. He first points out that if any link in the chain of causation is missing, then everything coming after the link will also be missing. It follows that, if the first cause is missing from the chain, then so will the second, third, and so on up to and including the most recent cause. But, he claims, if the chain of causation extends back to infinity, then there is no first cause — and so there would be no second, third, or any other cause. This means that the most recent cause — or what is going on right now — would also not exist, which is absurd. Therefore, the chain of causation cannot extend back to infinity. There must have been a beginning.

This is such a bad argument that it is surprising anyone ever fell for it. Aquinas is simply confusing (a) the existence of a cause that he calls "the first cause" with (b) its property of being first. That is, what we might label the "first" cause cannot be missing from the chain, otherwise the "second" cause would also be missing, and so on. That much is true. But it does not follow that this event that we're calling "the first cause" cannot have a predecessor. If the chain of causation is infinite, then there is no first cause. But no link in the chain will be missing on account of that.

One can also understand Aquinas's mistake this way: he is either equivocating or begging the question, depending on how the term "first cause" is interpreted at the beginning of this part of the argument. He is begging the question if by "first cause" he means the cause that has the property of being first, for then he is simply assuming the very thing he is attempting to demonstrate. If, on the other hand, he is merely naming a particular cause in the chain "the first cause", then he equivocates when he later uses the same term to mean the cause that has the property of being first.

Aquinas therefore has failed to show that the chain of causation must have had a beginning. As a result, his entire argument falls apart.

But even supposing that there was a beginning, it does not, of course, follow that there must be a God. There are, in other words, additional problems with Aquinas's argument. The first cause does not have to be a conscious being, much less one with all of the unusual properties commonly ascribed to God — it could be, for instance, the Big Bang. Nor does Aquinas's argument establish that the first cause is still in existence, or that there was only one first cause. In addition, it is no longer so certain that everything that occurs in the world around us must have a cause. Quantum mechanics has convinced most physicists that uncaused events can occur. For all these reasons, and others like them, the first cause argument should not be taken seriously.