Aristotle assumes, in his arguments for an Unmoved Mover in Physics VII and VIII, that all of the causes of motion that he is considering are non-successive, in the sense that the cause does not end before the effect ends. Why does he do so? Not because he thinks that all causes are non-successive. As we saw in the last post, Aristotle believed that causes can end before their effects end. So, why does he make the contrary assumption in Physics VII and VIII.
As I argued in previous post, Aristotle’s arguments cannot be based on the outdated idea that there is no inertia or no enduring impetus for motion in nature. Aristotle does not assume that all motion in the universe is mechanically connected to the heavenly spheres (like a clockwork mechanism) in such a way that, if the heavens stopped moving all the bodies in the universe would instantaneously lose their impetus to be moved or to move other bodies.
Instead, as I showed in the last post, Aristotle assumes that the passage of time depends on certain simultaneous motions, the primum mobile or prima mobilia. If the heavens stopped rotating, time itself would freeze. Without the passage of time, inertia and impetus are both rendered ineffective, since inertial motion is a time-dependent phenomenon. Heavy bodies, for example, move downward with a natural velocity that depends on their density (relative to the medium) and on the medium’s viscosity. But velocity is defined in terms of change of location per unit of time. If time does not progress, velocity alone is not sufficient to ground the body’s change in location.
What moves the prima mobilia? The first mover can’t be a time-bound agent. A time-bound agent is carried along with the flow of time. A mover or agent cannot act at an instant of time without acquiring the power to act at that moment of time. Having the power to act at earlier points in time is causally irrelevant. The power to act is a naturally persistent power. If the agent had the power to move the spheres at time T1, nothing happens to deprive it of that power, and time moves on to time T2, then the agent will still have the power to move the spheres at T2. However, its having that power at T2 depends on the arrival of time T2, which depends on the heavenly spheres’ reaching the appropriate location (L2). Thus, a timebound agent cannot account for the flow of time. Supposing that is responsible results in an impossible cycle of causal dependency: (i) the arrival of time T2 depends on the spheres’ reaching L2 (because time is only the measure of motion, and the movement of the heavenly spheres is by hypothesis the primum mobile), (ii) the spheres’ reaching L2 depends on the prime mover’s having the power at T2 to make the spheres reach L2 at T2, and (iii) the prime mover’s having the power at T2 to move the spheres depends on the arrival of time T2.
The only way to break this vicious cycle is to suppose that the prime mover is not timebound but timeless. A timeless agent possesses a timeless power to move the prima mobilia. A timeless agent does not have to work “within” time, waiting for the appropriate instant of time to arrive. Instead, the timeless agent creates (eternally) the structure of time itself, just as a human author or composer creates the temporal structure of a story or musical composition. Tolkien does not write during the Third or Fourth Age of Middle-Earth. His act of composition is outside the temporal structure of his books. It makes no sense to ask whether Beethoven composed his Ninth Symphony during its first, second, or third movements. Similarly, it makes no sense to ask when the prime mover caused the prima mobilia to move.
Why can’t the primum mobile cause its own motion? Because the primum mobile is timebound. Anything changeable or moveable is timebound. If the primum mobile were responsible for its own motion, we would still have a vicious cycle of causal dependency: the primum mobile‘s power to act at time T2 would depend on the arrival of that moment, which would depend on the primum mobile’s exercising that power at T2.
And now we can see why Aristotle assumes that all the timebound causes of motion are simultaneous with their effects. Causes that end before the effect ends cannot explain why time continues to pass after their own termination. Therefore, the total cause of a change occurring at an instant T cannot be a motion or process that ends before T, since action at a temporal distance is impossible. Motions that end before T arrives cannot be causally sufficient to explain the arrival of T.
This fact helps to explain why it is crucial for Aristotle’s argument that time be continuous. Before any instant there is a dense series of instants. No instant has an immediate predecessor. Every motion M that ends before T ends a finite distance in time before T, and this distance in time renders M impotent in relation to causing the primum mobile by which T’s arrival is actualized. Only an eternal agent, outside of time, can have the power to cause time itself to progress.
This interpretation also resolves the Gap Problem. Aristotle’s proof seems to establish only existence of an unmoved mover, but he immediately jumps (in Physics VIII, 6) to the conclusion that the prime mover is absolutely immovable, devoid of any passive potentiality. We can see now why Aristotle’s leap is justified. The prime mover cannot be a timebound agent. Only a timeless agent can break the cycle. And only an absolutely immovable agent, an agent of pure actuality, can be timeless. Something with any passive potentiality is changeable, and anything changeable is timebound.
Could the arrival of a moment of time T be explained by an infinite regress of motions, all occurring at T? Aristotle argues convincingly that such a regress is impossible. I will take those arguments up in my next post.
The Analytic Thomist 