12.1 Five arrangements of motion and rest in the universe (Book 8, Lessons 5-6)

Aristotle proposes five possibilities for the distribution of motion and rest in the universe and rejects four of them:

1) That all things be always at rest and nothing ever in motion.

2) That all things be always in motion and nothing at rest.

That some things are in motion and others at rest, in any of three ways:

3) that the things that are in motion are always in motion and those at rest always at rest, and nothing at one time in motion an at another time in rest,

4) that all things are apt to be in motion and to rest and that nothing is either always in motion or always at rest,

5) that certain things are always immobile and never in motion, others are always mobile and never at rest; still others may be in motion at one time and at rest at another.

Aristotle omits two more possibilities:

6) that certain things are always immobile and never in motion, while all others may be in motion at one time and at rest at another,

7) that certain things are always in motion, while all others may be in motion at one time and at rest at another.

(1) The first possibility, maintained by Zeno, Parmenides and Melissus, is against all evidence and challenges the first principles of natural philosophy. Now an error that affects all beings and all sciences is not to be reproved by the philosopher of nature but by the metaphysician, who considers first principles.

(2) The second possibility represents the opinion of Heraclitus, who said that all things are always in motion. It is also against the evident principles of natural science to say that there is no such thing as rest in the universe. But it is less evident that there is no motion than that there is no rest. For there are some motions so weak and insignificant that they can be scarcely noticed; for that reason it is easy to suppose that something is at rest when it really is not. But great and strong motions cannot be concealed; hence it cannot be said that the senses are deceived in perceiving motion as they are in perceiving rest.

Heraclitus was led to his doctrine as a result of considering growth. For he observed that a person grows a small amount in one year and, supposing that growth is continuous, he believed that in each part of that year he was increased with respect to part of that quantity; and yet that increase is not sensed, because it comes in a small portion of time. He reasoned, therefore, that the same thing happens in other things which seem to be at rest. Aristotle argues on the contrary that some growth is in spurts: nutriment slowly disposes the organism and at the right time a rapid multiplication of cells takes place; the same thing could be said of any motion of alteration. Besides, each of these motions has a final term, whether of growth or alteration, when the mobile rests in what it has reached. As for local motion, rest in a terminal (such as on the ground) is obvious.

(3) The third possibility is likewise against sense observation, that some things alternate between motion and rest. It also excludes compulsory motion, which is contrary to the natural rest of a thing, and generation and corruption, which suppose an alternation between existence and non-existence. Also it destroys all motion that goes from one term to another.

(4) The fourth view, that all things are apt to be in motion and to rest and that nothing is either always in motion or always at rest, can be refuted by establishing the existence of an immobile first mover, which is what Aristotle attempts to do next.

12.2 "Whatever is moved is moved by another." (Book 7, Lesson 1)

It is obvious that some things are moved by another, whenever the principle of motion is from without, such as in pushing, pulling, heating etc. This is termed "compulsory" motion. What is not obvious is how something which has within itself a principle of motion is moved by another.

Where anything moves itself, it is by reason of one part moving another, since mover and moved are mutually opposite as act to potency, as when an animal walks by movement of its legs (cf. Book 8, Lessons 10-11). If we ask further what moves the legs, we ultimately come to the brain, where the motor centre controlling the nerves and muscles is directed by the knowledge and desire of the animal's soul. What is impossible is that a solid whole should move itself, because something cannot be in act (imparting motion) and in potency (receiving motion) at the same time under the same aspect.

12.3 Gravitational movement (Book 8, Lessons 7-8)

What of gravitational motion, which is not compulsory but natural? First, gravitational fall is not self-motion, since self-motion is proper to animate bodies which can start or stop their motion and move one way or another according to appetite, whereas heavy bodies, as such, are not animate and must fall if there is no impediment, and they must fall down. More fundamentally, gravitational fall applies to solid continuous bodies (without flexible limbs), where there can be no division between moving and moved parts.

To understand gravitation, we must consider the different senses of "being in potency": One can be in potency to knowledge before he has learned something. But after he has learned it, but has another potency to think about what he has learned; to activate this potency (which is virtually in act) he needs no outside agent, but simply has to think, unless he is prevented by other occupations or by sickness or by his will. On the other hand, if he were not impeded and still could not think, then he would not be in the habit of science but in its contrary, namely ignorance. Likewise, the nature of the heavy is to have an aptitude to be down. Hence, to ask why a heavy thing is moved downward is exactly the same as to ask why it is heavy. Consequently, the generator is the per se mover of the heavy, whereas the remover of obstacles is a per accidens mover. The gravitational motion of heavy things is natural, because they have in themselves the principle of their motion, not indeed a moving or active principle but a passive one, which is a potency to such-and-such an act.

12.4 Magnetism and projection (Book 7, Lesson 3)

A magnet moves a piece of iron from a distance through an electric field set up in the medium encompassing it and the iron. The earth has a similar magnetic field that affects a compass; so does the electric motor. Thus magnetism operates by altering the molecular structure of the iron it affects and in this way pulls it.

Projection, however, Aristotle explains as a reverberation between the projectile and the air: the projectile pushing the air and the air coming from behind and pushing the projectile. In Book 8, Lesson 8, however, Thomas says: "When a ball rebounds from a wall, it is moved per accidens by the wall but per se by the one who first threw it. For it was not the wall but the thrower that gave it the impetus for motion, but it was per accidens that, being prevented by the wall from continuing according to its impetus, it rebounded into a contrary motion, the original impetus remaining." Again in De potentia (q.3, a.11, ad 5) Thomas says: "An instrument is understood to be moved by the principal agent as long as it retains the power of the principal agent impressed in itself; thus an arrow is moved by the archer as long as the power of the archer's impulse remains." Thus Thomas introduces the modern notion of impetus, which is a form, similar to heaviness (or knowledge in the above comparison) which accounts for the projectile's motion. This is an accidental form that acts in a similar way to natural heaviness, but is transient in that can be corrupted by resistance (or friction).

Thus everything moved is moved by another, but not always here and now, because both natural movement (whether gravitational or living self-motion from a soul power) and compulsory (as in a projectile) last in the mobile after the generator or mover has conferred on it a virtual motion.

12.5 There is no infinite series of moved movers (Book 7, Lesson 2)

We must keep in mind that in a series of moved movers, the last thing (which is simply moved) and the moved movers are being moved together, like coaches in a train, and the motion of each mobile is one and finite. Therefore there is a finite time for the motion of the whole series.

It could be argued that the motion of an infinite series would have to be infinite and therefore take place in an infinite time, since finitude and infinitude of mobiles, motions and time are correlative. But this argument is not conclusive, since it is possible that there be an infinite motion in finite time, so long as the motion is not one and the same but other and other, namely, as an infinite number of things are being moved. For there is nothing to prevent an infinite number of things from being moved at once in finite time.

The latter argument could be made efficacious if we assumed that the infinite number of moved movers were not separate mobiles, but one continuous mobile; in that case its motion would be infinite. One could object that it is impossible to the specific natures of most things for them be joined together in one enormous continuous body. Yet there is nothing opposed to this in the generic nature of a body. So the argument holds.

He gives another argument (Book 8, Lesson 9): It has been shown that whatever is being moved is being moved by another. But that by which it is moved is itself either moved or not moved; and if it is moved, it is either moved by another or not. But if there is something which is moved by another, it is necessary to come to a first that is not moved by another. This is because in an infinite series there is no first. But if the first does not act, the last does not act, and there will be no mover.

Looking at the process from a first mover, we can see that in a series of instruments moved by the hand, none of them would be active without the influence of the first.

12.6 A first unmoved mover (Book 8, Lesson 12-13)

If a mover is the first of a series, it necessarily follows that it not be moved by another per se, although it is possible it be moved by another per accidens, as by removing an impediment (like taking off the breaks).

It is possible for there to be a series of moved movers according to one motion where the first mover of this series is moved according to another motion. Thus a series of pistons belts and rods can move the wheels of a car, all according to local motion, but the pistons are driven by combustion of fuel, which is alteration and substantial change. But since the different kinds of motion, even when the series branches into another kind of motion it cannot be infinite. It is possible in a series to revert to the first kind of motion, but not in a circle to the same numerical starting point. Thus the combustion in a motor is controlled by the local motion of the driver's foot on the accelerator, and the motion begins with the driver's will.

Yet if we examine carefully such shifting between one species of motion and another, we find that we do not have a single series of moved movers in per se dependence on a first. Rather, at one step a mover may really be a per accidens cause of the subsequent movement. Thus the sending of fuel into the carburator is an accidental cause of combustion, since it merely supplies a condition for the natural motion of combustion. Aristotle uses the example of the motion of an animal, which can be traced per se to its soul, but per accidens to many external environmental factors.

The convergence of accidental causes assures the continued cycle of generation and corruption in the world, but Aristotle looks beyond these to a cause to account for the generation and perishing of things being continued forever. He says that this cause cannot be something itself perishable, since what does not exist forever cannot be the cause of what is forever perpetual and necessary. Accordingly, therefore, it is manifest that if there are a million principles that are movers and immobile, and if there are many things that move themselves, of which some perish and others come to be, and among these some are mobile and some movers, nevertheless there must be something above all of them that by its power contains all the things which would be the cause of the continual change affecting them by which they sometimes are and sometimes are not and through which these latter are the cause of coming-to-be and of motion for others, and these for yet others. For every generator is a cause of generation to the thing generated, but it is from some imperishable first principle that perishable generators possess the characteristic of being causes of generation. If, therefore, the motion through which some things at one time exist and at another do not is perpetual, as has been postulated, and a perpetual effect cannot exist except from a perpetual cause, then necessarily, the first mover is perpetual, if it is one; and if there are more than one first mover, they too are perpetual.

One perpetual principle ought to be posited rather than many, since finite principles ought to be preferred to infinite, and one first principle rather than many. For if the same effects happen or follow from positing finite principles as from positing infinite principles, one should assume that the principles are finite, and if one suffices that is better.

12.7 The first mover is perpetual and wholly unmoved (Book 8, Lesson 13)

If some principle is an immobile mover, yet moved per accidens (as a soul moved with a body), it cannot cause a continuous and perpetual motion. For the reason that animals do not always move is that they are moved per accidens (by environment). But it has been shown that the motion of the universe must be continuous and perpetual. Therefore, the first moving cause in the whole universe must be not be moveable even per accidens.

Aristotle has thus far argued for the immobility of the first mover from the perpetuity of motion in general. Now he tries to show that there is a single, continuous perpetual motion, which accounts for and is prior to all earthly generation and corruption. That he finds possible only in circular motion (Lessons 14-20), specifically in the motion of the stars and other heavenly bodies, which he supposed rotated daily around the earth as well as moving in some other elliptical paths. He argues (Lesson 13) that motion and rest occur on earth because of the differing positions that the sun (mainly) has with regard to the earth.

12.8 Analysis of the argument thus far

Aristotle has argued that in any series of moved movers we must come to a first unmoved mover. Any essentially subordinated series we can think of terminates either at a natural principle, such as the soul of an animal and the form of heaviness in any body, or at a quasi-natural principle such as impetus in a projectile, which is a power for motion comparable to heaviness, except that it is transient. Aristotle and Thomas are clear that natural motion, by definition, comes from an internal principle and not from any outside cause. Any dependence of an essentially subordinated series of movers on further causality is by way of accidental subordination, such as the chicken-from-the-egg series. Such dependency on a generator or an imparter of impetus goes into the past and can go on infinitely with no first mover.

How, then, can Aristotle maintain that this series of perpetual generation (on the assumption of the eternity of the world, as explained above) depends on an unmoved mover? Aristotle is not looking back into the past to account for the continuity of generation but for a mover here and now. In discussing both animal motion and the continuity of generation, he shows that all natural motion on the earth depends on environmental conditions. He does not enumerate them here, but they are the alternations of seasons, night and day, heat and cold, rain and dryness, wind etc. These are not causes, but conditions of natural motion, and in that sense accidental; nevertheless they are indispensable conditions for life and motion on this planet. Principally, these conditions boil down to the sun's alteration of the earth by heat and light and the local motion of the earthto correct Aristotleputting it at varying exposures to the sun.

Aristotle thought that the first motion responsible for all the other motions in the universe was that of the sphere of the "fixed stars" (embedded in the invisible sphere), which seems to rotate around the earth by an even and regular perpetual motion. This motion drags the spheres of the sun, moon and planets which, in addition, have their own proper motion which makes them deviate from the regular pattern of the fixed stars. The motion of each of these spheres, he says, but particularly the "first motion" of the fixed stars, requires an unmoved mover. Why is that? He goes on to explain:

12.9 The first mover must have infinite power and be immaterial (Book 8, Lessons 21-23)

Aristotle argues that it is impossible for anything of finite power to cause motion for an infinite time. But infinite power cannot exist in a magnitude (i.e. anything material). That is because any magnitude is finite and its power must therefore be limited. To the objection that an infinite power must move with infinite speed, Aristotle replies than any power which is not in a magnitude acts through an intellect, which can freely control its output to a finite speed.

Aristotle repeats that projectile motion cannot be perpetual and is not even continuous (although it appears to be) because the air set in motion in turn pushes the projectile and so on by a bouncing process called antiperistasis, until the motion dies out.

He then goes on to conclude that the mover of the "first motion" is immaterial, intellective and one. Thomas concludes: "And thus does the Philosopher in his general consideration of natural things terminate at the first principle of the whole of nature, Who is the One above all things, the ever blessed God. Amen."

12.10 Critique of this argument

Thomas' conclusion that this first mover is God jumps over Aristotle's further development of this argument in Metaphysics XII, where he maintains that the efficient mover of the first motion (maybe the soul of the star sphere) is itself moved by a completely separate intelligence in the order of final causality, i.e. as an object of love.

Aristotle had no idea of impetus or of the corresponding idea of inertia, which explains why, once the earth was set in motion, it keeps on spinning on its axis and rotating around the sun indefinitelyuntil what negligible resistance it encounters in the rarefied place called space eventually stops it. Just as a satellite requires energy to be put in space, but once it is there keeps on going without any more fuel, so the earth (and other planets and heavenly bodies in their own paths) goes on its way without any need of refuelling or additional push. Thus, if we graft the notion of impetus onto the Aristotelian system, there is no need to posit a spiritual mover of infinite power to explain heavenly motion.

Nowhere does Thomas disagree with Aristotle's argument. Even though we saw above that he ventured the concept of impetus, he never applied it to the motion of heavenly bodies. Thus he never questioned the assumption that, given the eternity of heavenly motion, there must be an immaterial mover of infinite power. And if, according to Christian faith, the world had a beginning in time, for all the more reason must there be an immaterial creator of infinite power. That is why he said that the way of proving God's existence by motion is "more manifest" (Summa theologiae, I, q. 2, a. 3).

Thomas does, however, speak of different "ways" of proving the existence of God. These are not so much different proofs, but they look at different aspects of the potentiality and mobility of things, showing their dependence on an immaterial agent. In these ways Thomas never for once doubts Aristotle's universe or his "proofs", but his perspective goes beyond them to more solid ground.