In the world there are things that are in motion, and whatever is in motion must have been moved by something else. According to Aquinas, this chain of movement cannot go back to infinity. There ‘ must have been a first, or Prime, Mover, which itself was unmoved. The Unmoved Mover began the movement in everything without actually being moved. Aquinas argued that the Prime Mover is God: 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 can be moved except it is in ‘ potentiality to that towards which it is moved; whereas a thing moves inasmuch as it is in act. For motion is nothing else than the reduction of something from potentiality to actually, except by something in a state of actuality. Aquinas was speaking of motion in the broadest sense.
He included not only movement from one place to another, but also movement in the sense of change of quality or quantity. According to Aquinas, an object only moved when an external force was applied to it. He continued that objects only changed because some external force had brought about the change. He spoke of filings achieving their potential through an external influence. Aquinas used the example of fire making wood hot. When applied to wood, fire changes the wood to achieve its potential to become hot.
In order for a thing to change, actuality is required. If it were not, a thing would have to initiate change in itself, which would require that it was both actual and potential at the same time. Aquinas considered this to be a contradiction. For example, if wood could make itself hot then it would be hot already. Wood cannot be hot to begin with; otherwise, it would not change and become hot. The fact that it is not hot already is its actuality. The fact that fire can make it hot is its potentiality.
In turn, something must have made the fire change and become alight. Each change, therefore, is the result of an earlier change. Aquinas, however, did not accept that there was a series of infinite changes. He concluded that there was a point at which the first movement (or change) occurred, brought about by ‘a first mover’. Therefore, according to Aquinas, ‘it is necessary to arrive at a first mover, moved by no other; and this everyone understands to be God’.