Here is the text of the First Way (from the Leontine translation, with a few minor changes by the authors, set in italics):
The first and more manifest way is the argument from motion. It is certain, and evident to our senses, that in the world some things are in motion. Now whatever is moved is moved by another, for nothing is moved unless it is in potentiality to that towards which it is being moved; whereas a thing [actively] moves [something] inasmuch as it is in act.[1] For motion is nothing else than the reduction (Latin educere) of something from potentiality to actuality. But nothing can be reduced (reduci) from potentiality to actuality, except by something in a state of actuality. [As, for example,] fire makes wood, which is potentially hot, to be actually hot[2], and thereby moves and changes it.
Now it is not possible that the same thing should be at once in actuality and potentiality in the same respect, but only in different respects. For what is actually hot cannot simultaneously be potentially hot; but it is simultaneously potentially cold. It is therefore impossible that in the same respect and in the same way a thing should be both mover and moved, i.e., that it should move itself. Therefore, whatever is in motion must be put in motion by another.
If that by which it is put in motion be itself put in motion, then this also must needs be put in motion by another, and that by another again. But this cannot go on to infinity, because then there would be no first mover, and, consequently, no other mover; seeing that subsequent movers move only inasmuch as they are put in motion by the first mover; as the staff moves only because it is put in motion by the hand. Therefore, it is necessary to arrive at a first mover, put in motion by no other; and this everyone understands to be God.
We must make one important linguistic observation at the outset: Thomas’s word movere (and the Greek word kinein to which it corresponds) had a much wider application than modern English ‘move’. Not just locomotion (change in place) but any kind of intrinsic change counts as motion. Motion includes locomotion, change in volume or mass (growth or diminution), and change in quality.
As we shall see, the structure of this argument is very similar to that of the Second Way. In both cases, we have a domain of facts or events and a cause-and-effect relation on that domain. Thomas assumes that every element of the domain has a cause. The transitive closure (‘cause*’) of the cause-effect relation is the smallest relation that contains every instance of cause-and-effect and that satisfies the principle of transitivity: if x causes* y, and y causes* z, then x causes* z. St. Thomas assumes that every chain of causes* is finite.
Nonemptiness. There are changes, i.e., states that constitute a thing’s being changed in some way.
Seriality. Whenever there is a change, there is some state that causes the changed state.
Finiteness. The transitive closure of causation, cause*, cannot be backwards infinite.
That is, there are no causal loops and no infinite causal regresses. Consequently, the cause* relation has the following formal characteristics:
• Transitivity. If x causes* y, and y causes z*, then x causes*z. By definition of ‘cause*’ as a transitive closure.
• Seriality. For every x in the domain D, there is a y such that x causes* y. By definition of ‘cause*’ as the closure of causation.
• Asymmetry. If x causes* y, then y does not cause* x. If x caused* y and y caused* x, then there would be an infinite chain: x causes* y, y causes* x, x causes* y, etc.
• Irreflexivity. For no x does x cause* x. For the same reason.
• Well-foundedness. Every non-empty subset of the domain has at least one element that is not caused* by any other element in that subset. No infinite regresses anywhere.
From these formal properties, it follows logically that there is at least one entity not in the domain D that is the cause of some members of D. This is an uncaused cause or unmoved mover.
Theorem 1.1. There is an actually existing first cause of changes which lies outside the domain of changes, and every change is caused* by some such first cause.
This is a simple and logically sound argument. Nonetheless, there are four serious problems with it, which I will lay out in my next post. Only a radically new interpretation of the First Way can overcome these four problems.
[1] The Leontine translation: “Now whatever is in motion is put in motion by another, for nothing can be in motion except it is in potentiality to that towards which it is in motion; whereas a thing moves inasmuch as it is in act.” The translators confusingly use different English phrases for the single passive verb, movetur (is moved).
[2] The Leontine translation: “Thus that which is actually hot, as fire, makes wood, which is potentially hot, to be actually hot.” The Latin does not mention the actual hotness of the fire: “ut ignis, facit lignum, quod est calidum in potentia, esse actu calidum.”
The Analytic Thomist 