With this lesson a new kind of intellectual adventure begins an we dig into St. Thomas Aquinas's famous "Five Ways" of proving the existence of God. We start with the argument Aquinas considered to be the "first and most manifest" --- the argument from motion or change.
Let me state the argument all at once. After that we'll swing back to explain, elaborate and illustrate.
(a) We know from experience that some things are in motion ("motion" taken my Aquinas to refer not only to movement from one place to another but to any state of change). (b) Motion or change, Aquinas explains, is the "reduction of something from potentiality to actuality." (c) Nothing can be moved from potentiality to actuality "except by something already in a state of actuality." Said another say, "Whatever is in motion must be put in motion by another." (d) If what puts something else in motion is itself in motion, then it also must have been put in motion by yet something else.
(e) This series of "movers" cannot go back infinitely, because then there would be no first mover, and if there were no first mover, there would be no other movers because "subsequent movers move only insomuch as they are moved by the first mover..." (f) Therefore, "it is necessary to arrive at a first mover, put in motion by no other; and this everyone understands to be God" (Summa I.2.3).
OK, I understand your initial reaction. Let's go though this more slowly...
Potentiality and Actuality
What in the world does Aquinas mean when he defines "motion" or "change" as "the reduction of potentiality to actuality"? Sounds complicated, but it's really not so much.
Think about the dashboard in my car. When I climb into my car on an August morning here is sunny Southern California, the dashboard in not "actually" burning hot. It has the "potential" to be burning hot (believe me) but it isn't. In order for my dashboard to move from being "potentially" burning hot to being "actually" burning hot, a change will need to take place. In this particular sense, a "reduction of potentiality to actuality" must take place. This is change. Motion.
And my dashboard cannot reduce it's own potentiality to be burning hot to actuality. It cannot "move" itself. It cannot give to itself what it does not have. What is needed is for something that is not merely "potentially" burning hot, but "actually" burning hot, to "act" upon it. Like maybe the sun.
Now, if the sun were only "potentially" burning hot and not "actually" burning hot, it could not bring my dashboard from the state of "potentially" to "actually" in respect to its becoming burning hot. In that case something else that was already "actually" burning hot (or had the inherent power to move the sun from being merely "potentially" burning hot to being "actually" burning hot) would have to first act on the sun bringing it from potentiality to actuality before the sun could act on the dashboard and move it from being merely "potentially" burning hot to being "actually" burning hot.
Now, If this is not as bright and clear as the summer sun in sunny Southern California, then either I've missed by true vocation or you've watched too many episodes of Trailer Park, or whatever the shows are called this season: Tadpoles in Crisis, Born to Eat, whatever
OK, so we understand the idea of change or motion in terms of potentiality to actuality. And we understand that in this sense nothing can move itself.
We can see this even more clearly by using an illustration that involves actual movement. Imagine a boxcar filled with hobos and moving down the track. It's moving because the boxcar immediately behind it (also filled with hobos) is moving and pushing the one on front of it. But that boxcar is also moving and therefore pushing the boxcar ahead of it only because the boxcar immediately behind it (filled with hobos) is also moving and pushing it. And again that boxcar is moving because yet another immediately behind it (more hobos) is also moving... and so on and so forth.
Notice that this entire chain of boxcars is moving at the same time, simultaneously. And while each of the boxcars (obviously) has the "potential" to be moving, none of them has the power to move itself. In order for them (and the hobos within) to "actually" move, the entire chain must be acted upon by something that "actually" moves. At the bottom of this series of hobo-filled boxcars there must be something that has the power of movement within itself, like an engine. There must be a "first mover." When it moves, all the boxcars and all the hobos move simultaneously.
Similarly, Aquinas argues, in our universe of moved movers, none of which have the power to move themselves, there must be a "first mover" in whom the power of movement simply exists. An "unmoved mover." A being in whom there is no potentiality that needs to be actualized by something or someone else. A being who is pure "act," pure "actuality." And this being we call God.
An Infinite Series of Movers
Now, the most common objection to the argument outlined above is to simply assert that the chain of movers is infinite. "In our universe comprised of moving things, all in turn being moved by other things, each in turn being moved by still other things, the series simply goes back forever. There is no 'First Mover'. Rather what we have is an infinite series of moved movers."
To understand precisely how this is impossible (yes, I said "impossible"), we have to introduce a distinction at this point, because Aquinas talks about more than one kind of causal series.
First, there are causal series that are ordered per accidens or "accidentally." This is where the members of the causal series are connected, but where the activity of each member is not "directly" and "immediately" in the "here and now" dependent on the activity of prior member of the series.
For instance, my grandfather begat my father and my father begat me and I begat my son. And while it's certainly true that each member of this causal series is dependent on previous members for his existence (keep that in mind, young man) we are each independent of our fathers in the sense that our power to act is not immediately dependent on them. My father has been dead seventeen years and yet I can sit in Starbucks this morning and write this lesson. And while I'm sitting here typing away, my son is getting on an airplane to fly to New York on business of a dubious nature I won't go into.
This is an example of a causal series ordered per accidens. This kind of causal series is ordered horizontally in the sense that it extends backward and forward through time. We could argue whether such a causal series could at least be conceived as infinite.
But then there's another sort of causal series, one that is ordered per se or "essentially" --- the kind illustrated by the boxcars and hobos. In this kind of causal series the members are ordered vertically in the sense that the movement of each member of the series is entirely dependent "in the here and now" on the movement of the member prior to it, and so forth to the beginning. This is the kind of causal series Aquinas is talking about in his argument from motion, and in this kind of causal series there must be a first mover. It is impossible for the series to be infinite.
To illustrate, think of a boy using a stick to move a dead rat which is moving a half-eaten sandwich across the ground. If mom calls the boy in for lunch and he drops the stick, both the dead rat and the half-eaten sandwich stops moving. The entire series comes to a halt.
And the reason is that the movement of the half-eaten sandwich was immediately dependent on the movement of the dead rat, which was immediately dependent on the movement of the stick, which was immediately dependent on the movement of the boy's arm. In fact, strictly speaking the half-eaten sandwich was never really being moved by the dead rat, and the dead rat was never really being moved by the stick. The entire series was bring moved by the boy, and so the instant he dropped the stick to run inside, the series of "moved movers" no longer existed.
Aquinas said, "It is clear that when a thing moves because it is moved, the mover and the mobile object are moved simultaneously." Because of this, in the kind of series Aquinas is talking about, if there is no "first mover" there are no other movers in the series, because "subsequent movers move only insomuch as they are moved by the first mover..."
In this kind of series, if you eliminate the "first mover" you eliminate the entire series.
What about human beings?
"But what about human beings and other living things? Clearly, we have the ability to move ourselves. For instance, I can sit at a piano and move my fingers on the keys and play whatever I like. There's no "essential" causal series here leading back to an unmoved mover. It's just me."
Yes, but your finger's potentiality for motion is actualized by the muscles in your hands. And the muscle's potentiality for movement is actualized by your nervous system. And the potentiality of your nervous system is actualized by its molecular structure. And the potentiality of these molecules is actualized by their biochemical make-up. And the potentiality of the biochemical make-up of those molecules is actualized by their atomic and sub-atomic structure and...
To quote philosopher Edward Feser:
That the molecules composing the nervous system constitute a nervous system specifically amounts to their having a certain potency which is here and now actualized; that the atoms composing the molecules constitute just those molecules amounts to their having a certain potency which is simultaneously actualized, and so on.
When you move your fingers over the keys, an entire chain of potentialities being reduced to actualities is taking place simultaneously, all at once and in the here and now.
And since, as St. Thomas said, nothing can be moved from potentiality to actuality "except by something already in a state of actuality", since "whatever is in motion must be put in motion by another", there must ultimately be a first mover. And this first mover must be a mover that is not moved, an actualizer who needs no actualizing, a being in whom there is no potentiality, a being who is pure actuality. Pure act. A being who exists through the power of his own essence. A being who sounds very much like the one who said to Moses at the burning bush, ""I am who am. I am the one who exists."