Aquinas begins his first way with an observation of what he saw around him. That observation was that:
‘Some things are in motion.’
Aquinas did not only mean things that are moving, as a ball might move across a pool table. What he meant by motion is closer to what we might call ‘change’. So when he says “Some things are in motion”, he means “Some things are changing” or “Some things change”. However, since motion is the word Aquinas uses, it is the one we must also use.
So, some things are in motion, but what exactly is this motion? Aquinas defines motion is something moving from potentiality to actuality. So motion is when something goes from being potentially one thing to actually being that thing. Aquinas’ example concerns a piece of wood.
A piece of wood is cold, actually cold. However, it is potentially hot, in other words, it could move to be hot, it could change and become actually hot. So in this instance, the piece of wood would move from being potentially hot to actually hot.
The question is, how does this motion occur? Aquinas realised in order to move something from potentiality to actuality, we need something in actuality. In order to move something from potentially hot to actually hot, we need something that is actually hot. But what is this something in actuality?
Aquinas considers two possible answers. Firstly, that the thing moves itself and secondly that it is moved by something else.
Let’s look at the first of these answers. Could something move itself? Could the piece of wood move itself from potentially hot to actually hot?
Aquinas says that this is not possible, that nothing can move itself from potentiality to actuality. But why? In order to move something from potentiality to actuality, we need something in actuality. If something moved itself, it would need to be both in potentiality and actuality at the same time and the same respect. Put it this way, in order for a potentially hot piece of wood to move itself to actually hot, it would have to be both potentially hot and actual hot at the same time. Put simply, the piece of wood would have to be both not on fire and on fire at the same time. Clearly this is not possible.
So Aquinas concludes that for something to be moved from potentiality to actuality, we need something else in actuality. For example, in order to move the piece of wood from potentially hot to actually hot, we need something else that is actually hot, such as a lit match.
We have now established what is called a regressive sequence, in this case, a regressive sequence of movers.
A regressive sequence is a sequence that can traced backwards. So the wood was moved by the lit match, which was moved by the hand, which was moved by the muscles in the arm, which were moved be a desire to start a fire, which was moved by feeling cold and so on. We can trace each movement backwards to the thing that moved it. But how far does this sequence go?
One thing is moved by something else, which is moved something else and so on. Does this go back infinitely? Can this sequence go back ad infinitum?
Aquinas argues that it cannot – the sequence cannot go back infinitely. He rejects the possibility of an infinite regress of movers and clams the sequence must be finite, or put another way, there must be a first mover.
But why must there be a first mover?
Aquinas argues that if there were no first mover, there would be no second mover and no third mover and so on, meaning there would be no movement now. However, we clearly started the argument tight he observation that there is motion.
If there were no first mover, there would be no subsequent motion, no following motion. Since there is motion now, we should conclude that there is a first mover, which was not moved by anything else.
Aquinas finishes by stating that this unmoved first mover is God.
P1 Some things are in motion from potentiality to actuality.
P2 Motion from potentiality to actuality must be by something in actuality.
P3 Nothing can move itself from potentiality to actuality.
P4 Motion from potentiality to actuality move be by something in actuality.
P5 This regressive sequence of movers cannot continue ad infinitum.
C1 There is an unmoved first mover.
C2 This is God.