- Whatever changes must be changed by something else.
- The causal series can't go on to infinity.
- Therefore, there must be a first mover.
- The first mover must itself be pure actuality: unmoved, unchanging, and unmovable.
- Therefore the first mover is God.
Recall that in the discussion of local motion, Feser abandoned (1.), and replaced it with something like "Some things that changed are changed by something else." This is fatal to his argument, though. Here's how he argues for (4.) (p. 95-96, for those of you following along at home):
Now, a first mover in such a series must be itself unmoved or unchanging; for if it was moving or changing - that is, going from potential to actual - then there would have to be something outside it actualizing its potential, in which case it wouldn't be the first mover.
This obviously relies on the idea that everything that changes is being changed by something external to it. If only some things that change require an external changer, then the argument is just a non sequitur.
Let's back up, though, and look at his argument for (2.) Feser says that Aquinas distinguishes between two types of causal series: accidentally ordered series and essentially ordered series. In an accidentally ordered causal series, the cause could have happened some time in the past: it need not be present to produce the effect. His example is a father, who was the "cause" of his son. The father need not be still alive, though, for the son to go on existing. In an essentially ordered series, the cause must be there for the effect to occur. The example is of a hand that pushes a rock using a stick. If the stick were not touching the rock, it would not be moving it, and if the hand were not pushing the stick, it wouldn't be moving the rock. Feser chases the chain of causation to deeper and deeper levels: the hand's motion depends on the firing of neurons, which depends on the biochemistry of the cell, which depends on the atomic structure of the molecules, which depends on the fundamental forces that govern particle interactions.
An accidentally ordered causal series could go on to infinity, says Feser. But not an essentially ordered series. (Thus, (2.) only applies to essentially ordered causal series.) The reason is that in an essentially ordered series, the later members only exist because the earlier ones do: the stick's push against the rock is only there because of the action of the hand. Feser says, "...it is only the first member which is in the strictest sense really doing or actualizing anything." He gives the example of a series of freight cars pulling out of a train station: the car you see is pulled by the one in front of it, and that one by the one in front of it, and so on. But they can't be all freight cars; somewhere up there there must be an engine car initiating the whole series.
What is needed is an appeal to something that does have the power of movement in itself, such as an engine car.
There are two problems with all this. First, the idea that causes can be arranged in nice ordered chains as envisioned by Feser just doesn't accord with what we know about the way the universe works.
The example of the train engine is what Dennett calls an "intuition pump." It's an example designed to make you think, "Of course there has to be a first member of the series." Here's a different example to pump your intuition a different direction: Think about two masses, A and B, in circular orbits around their common center of mass.
The change in A's velocity is caused by B's gravity, and the change of B's velocity is caused by A's gravity. There's no need for one of these to be causally prior to the other.
Instead of nice ordered chains of causality, there are complex interconnecting webs of interactions. When all the causes are present at once, as they are in the situations Feser calls "essentially ordered chains," no one of them need be considered the "first."
But even if we accept the idea that all causal chains are ordered, there is another problem from the physicist's point of view: everything in the world has the "power of movement in itself." Feser has slipped into talking about "movement," but recall that by this he really means "change." Changes, for the physicist, are the result of forces, and forces are part of the nature of those fundamental particles from which everything is made. It is the nature of massive particles to exert the gravitational force, of charged particles to exert the electromagnetic force, and so on. So any particle (or collection of particles) can be the "first mover" in the series.
(This post was written before I saw Feser's second response to my series. I will address his responses in a later post.)