Here is our translation of the first section of Aquinas’s First Way (from the Summa Theologiae I, Q2, a3):
The first and more manifest way is the argument from motion (change). 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 some state of actuality. For motion is nothing else than the reduction of something from potentiality to actuality. But nothing can be reduced from potentiality to actuality, except by something in a state of actuality. As, for example, fire [which is in some state of actuality] makes wood, which is potentially hot, to be actually hot and thereby moves and changes it.
This paragraph introduces three premises:
(1) Change (“motion”) occurs.
(2) Every change is the reduction of some potentiality to actuality.
(3) Every such reduction must be caused by some “mover” that is (at the time in question) in some state of actuality. I.e., mere potentialities cannot cause other potentialities to be reduced to actuality.
There is a considerable amount of philosophical substance built into the second and third premises, based on Aristotle’s analysis of change in Physics 3, where Aristotle defines change (kinesis) as the actuality (entelechia) of potentiality (dynamis) qua potentiality. According to Aristotle, there are two ways for things and states of things to exist: actually and potentially. Whenever a change occurs, a new thing or state of a thing appears in the realm of the actual, but it does not appear from sheer nothingness. Instead, it emerges from a prior condition of mere potentiality. In this way, Aristotle meets the challenge of the 5th-century BC Greek philosopher Parmenides, who argued that it is impossible for something to come from nothing, since we cannot even speak or think of something unless it exists in some way.
In addition, in Physics 4.11, Aristotle defines time as the measure of change. This means that he cannot define changein terms of time. For example, he cannot define a change in a thing as the thing’s being one way at one time and another way at a later time (the ‘at-at’ definition of change). Aristotle needs the notions of actuality and potentiality to provide an alternative. He introduces two kinds of actual entities: things and processes (kineses). Things are fully actual while processes are actual only qua potentialities, having an “incomplete” kind of actuality. For example, if Homer is at the 50 yard line and is running toward the end zone, then his location at the 50 yard line is actual simpliciter, while his process of running is actual qua a set of future positions that are merely potential and yet primed for actuality. Although only potential in themselves, they have (through the actual existence of a process) a kind of adjacency to full actuality.
Processes have a natural tendency to “unfold”, to proceed. As they do so, time passes. Time is the measure of the degree to which this or that process has successfully unfolded into a fuller state of actuality.
Aristotle’s account of time as involving the continuous unfolding of processes has a number of advantages over the alternative, Block-Universe theory of time, in which time is a fourth dimension modeled on the three dimensions of space. First of all, Aristotle’s account of change treats change as something real and different from the static variation of qualities over space. The Block Universe model treats change from cold to hot as a thing’s being cold at one “place” in time and hot at another, which, as J. M. E. McTaggart pointed out, essentially treats change as a kind of illusion. Second, since every process has an essential direction from an initial state that is fully actual to succeeding states in a condition of mere potentiality, there is an intrinsic and fundamental direction to time. All Block-Universe models struggle to account for that.
As a process unfolds, its successive stages undergo a transition from potentiality to actuality. As Aquinas asserts, such transitions require a cause in a condition of actuality. Causes are actual things and events. Each process has a natural disposition to unfold in a certain way and not others, but no process actualizes itself.
This leads us to the second section of Aquinas’s argument:
“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 [merely] 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 [strictly speaking] move itself. Therefore, whatever is in motion must be put in motion by another.”
A process consists in a set of potential states, states which are primed for actuality but lack actuality in themselves. If time passes, and nothing interferes, then the stages of the process will successively acquire actuality, but the explanation for this progression must come from something actual, an actual agent beyond the process itself. Processes cannot pull themselves up to successively greater actuality by their own bootstraps.
The unfolding of one process can actualize the unfolding of another. For example, a log’s burning can cause the progression of a process of heating a pot of water. We can say that the burning process propels the water-becoming-hotter process. The propelling process must be unfolding simultaneously with the propelled process. There can be no action at a temporal gap: a process occurring earlier in time cannot immediately propel a process occurring later, nor vice versa. This propelling relation is a causal relation. Consequently, it is transitive: if process A propels B, and B propels C, then A propels C. Since no process can propel itself forward, we can rule out vicious cycles of causation.
The next stage in the argument asks whether we could have an infinite regress of such processes:
“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.”
For time to move forward, reaching any arbitrary moment T, some actual change must occur at T. Some process already in motion before T must reach a stage of completion that consists in the stage corresponding to T becoming actual. To fully explain time’s “arrival” at T, there must be at least one primal change occurring at T: a change not actualized by any other change at T. These primal changes must be caused by something, but they cannot be caused by any event in time (including the actualization of any stage of a process), since they cannot be caused by events that are earlier, later, or simultaneous with them. There must be a non-temporal cause, not belonging to any process. This is Aristotle’s Prime Mover.
But must there be primal changes at each moment of time? Why couldn’t the changes actualized at T form an infinite causal regress, with each change caused by a simultaneous but causally prior predecessor, with no non-temporal cause ultimately responsible for any of them? Why couldn’t there be an infinite regress of processes, each simultaneously propelled forward by its predecessor?
In response, we have to follow Aquinas in distinguishing between two kinds of infinite regress: essentially infinite and accidentally infinite. An accidentally infinite regress is caused as a whole by some “first cause” outside the regress. An essentially infinite regress has no such external cause. Accidentally infinite regresses pose no problem for Aquinas’s argument: an accidentally infinite regress of changes occurring at T would require a non-temporal cause, a Prime Mover outside of time. Consequently, Aquinas must argue that an essentially infinite regress is impossible. Every infinite regress must have an external cause.
If we mentally “take away” the non-temporal cause of the changes at T, we are left with no explanation of why any change takes place at that time—or at any other time. An essentially infinite regress can explain why a particular change occurs, given the existence of causally prior changes, but it can’t explain why there is some change rather than no change at all. Take away the first, non-temporal cause, and you “take away” all the temporal causes as well. We are left with no causal explanation of the arrival of moment T, and so no explanation for the forward movement of time.
If changes occur at T, whether related to each other in an infinite regress or not, we can lump them all together into one gigantic Change-at-T and ask what caused that change to transition from potentiality to actuality. This requires a non-temporal or “eternal” cause.
So, the forward motion of time requires a Prime Mover outside of time. But what does this have to do with the existence of God? Aquinas concludes:
“and this everyone understands to be God.”
But isn’t there a big gap between being a non-temporal agent and being God—an all-powerful and omniscient spirit? This apparent gap led later scholastic thinkers to drop the argument from their natural theology, assigning it to the philosophy of nature rather than to metaphysics.
Aquinas recognized that he had not yet established the existence of something with all God’s divine attributes. The next 19 questions, comprising over 100 articles, do that. But he did believe that each of the Five Ways proves the existence of a being possessing at least one attribute that only God could possess.
To explain why time passes, we must posit a powerful agent outside of time. Aristotle discussed what it means for something to be “outside” time in Physics 4.12: a being outside of time must have a nature that excludes the possibility of its undergoing any intrinsic change. In other words, the Prime Mover must be not only unchanging but unchangeable. A thing perpetually at rest but movable or changeable is still in time. Only an unchangeable being can be outside time, and only a being outside time can cause the forward motion of time.
Once we know that God is unchangeable by nature, many other attributes follow. An unchangeable being must be pure actuality, with no intrinsic potentialities. Any physical thing or any thing with spatially located parts is capable of undergoing intrinsic change. So, God must be non-physical, with no spatially located parts. God is neither a particle nor a field, nor composed of particles or fields. God must be absolutely simple, with no parts of any kind, and God cannot be part of any larger whole. God must be immutable and impassible. He must be in some sense a necessary being, with no potentiality for non-existence. He is clearly “supernatural” by any reasonable definition of that word. The First Way decisively refutes naturalism.
