1. Plan and Exact Purpose of Book VIII
In Book VIII Aristotle's purpose is not only to demonstrate the prime mover. He also sets out to determine the distribution of all essential movers and movables according to their respective motion and rest. This adds to the complication of the book. For, in addition to a prime movable, a primum mobile, which is eternally moved, he must also show that there are movables which are only sometimes moved and therefore sometimes at rest. This general theme is effectively expounded at the beginning of chapter 3 and at the conclusion of chapter 9.
From this perspective, then, the proof falls into three distinctive parts:
1) Preliminary demonstration: the eternity of motion (chaps. 1-2).
2) Principal argument: layout of the world on the basis of movers and movables (chaps. 3-9).
3) Corollaries: properties of the prime mover (chap. 10).
2. The Eternity of Motion
For the eternity of motion Aristotle adduces two principal arguments:
a) A movable being is either eternal or generated. If generated, the generation, which is a change of one thing to another, presupposes anterior motion, and so on ad infinitum. If, on the other hand, we say that the movable is eternally pre-existent, then rest is prior to motion, an impossibility because rest is the privation of motion, hence presupposes it. Thus we must conclude that movables are generated, and this indefinitely or eternally without a beginning in the process, since one generation presupposes anterior motion, and this motion yet another, and so on in ceaseless regression.1 By similar reasoning Aristotle excludes the existence of an ultimate term in the process of change and becoming.
b) Besides, argues Aristotle, the eternity of motion follows from the eternity of time. If, like him, we accept as a demonstrated fact that time is eternal, it does indeed follow that motion too is eternal, since time, the number of motion, does not exist without motion.
3. Allocation of Motion and Rest and Demonstration of the Prime Mover
a) Presentation of the problem.2 - Various suppositions can be advanced concerning the state of rest and motion:
- either everything is always at rest,
- or everything is always moved,
- or some things are moved, others at rest.
Assuming the last supposition correct, we find that it in turn allows of three different possibilities:
(i) either that things moved are always moved, and things at rest are always at rest,
(ii) or that everything is alternately moved and at rest,
(iii) or that some things are eternally motionless, others eternally moved, and still others admit of both states, i.e., are sometimes at rest and sometimes in motion.
Possibilities (i) and (ii) must be rejected, as experience shows that: 1) not everything is at rest, 2) not everything is always in motion, 3) there are things which are sometimes moved and sometimes at rest. Remains to be proved, therefore, that possibility (iii) is the right one.
b) Whatever is moved, is moved by another.3 - Surprisingly enough, Aristotle does not here try to prove this principle in a priori fashion; he does it inductively, considering in how many ways a thing's motion may be brought about.
Leaving out of account accidental motion,4 there are, he observes, three ways for a thing to be moved:
- to be moved by nature and at the same time by itself,
- to be moved by nature without being moved by itself, and
- to be moved contrarily to nature, hence most clearly by another.
This list is exhaustive. In the first case the distinction of mover from moved is less obvious than in the other two, yet in all of them, as Aristotle explains at some length, what moves and what is moved are different. So, exploring the possible ways of being moved, Aristotle finds that all the facts in the matter come to one conclusion: whatever is moved, is moved by another.5
c) Necessity of a prime mover; this mover is immovable, eternal, one.6
Necessity of a prime mover. - Aristotle supplies a variety of arguments in proof of a prime mover, but all of them come to this: If everything moved is necessarily moved by something else, there has to be a first mover that is not moved by another. It is impossible, so runs the thread of all the arguments, that a series of movers which themselves are moved by another should be infinite, as it must be without a first or prime mover. For, if everything moved is moved by another, then this other is unmoved, or it is moved by still another, and so on indefinitely. But the series cannot be infinite, because in an infinite series there is no first, hence no second, no third, etc., and all motion ceases. So, the series must stop somewhere - anagke stenai -, says Aristotle, and never did two words leave more impress on the course of philosophical history.
Accordingly, the whole proof of the prime mover rests on the impossibility of an actually infinite series. Note well, however, that the series of movers is thought to be in essential subordination to one another. The argument does not hold for a series that is only accidentally connected. St. Thomas, at any rate, seems to think that such a series could be infinite.7
Immovable. - The prime mover is not moved by another. Either, then, it is immovable and utterly unmoved, or it is moved by itself. If moved by itself, it must be composed of one part which acts as unmoved mover and another part which is moved. Hence, whichever the case, there exists a first unmoved mover.
Eternal. - The prime mover is eternal. Aristotle deduces this from the premise, which he regards as certain, that motion is eternal. The Christian, in passing, could not of course argue from this ground.8
One. - There is only one prime mover rather than many. One prime mover, says Aristotle, is enough to account for the facts of motion. A plurality of prime movers would also explain them, but wherever two or more explanations are possible preference should go to the simpler. The simpler in this case is the unicity of the prime mover. Other considerations, continues Aristotle, point to the same conclusion, as the fact that motion, if presumed eternal, must be continuous, hence one. But it will only be one if the mover and movable involved in it are each of them one.9
d) Necessity of a first movable, primum mobile.10
- Some things, as remarked earlier, are sometimes in motion and sometimes at rest. Also now established is the existence of a prime mover, immovable, eternal, one. On these two premises Aristotle reasons the existence of a first movable that is perpetually in motion.
The prime mover, according to the argument, always imparts one and the same motion, and always in the same manner, since its relation to the movable never varies. Hence the prime mover cannot be the immediate source of variation in change and motion, such as we see in the constant succession of generation and corruption. But a perpetually moved mover can perform this function. By its own eternal motion this moved mover accounts for the eternity of the process of generation and corruption, and by its different positions it explains the alternating cycles - the point at issue - we observe in generation and corruption. At the same time this moved mover is uniformly moved by the first mover.
Briefly, Aristotle's scheme of cosmic motion requires both an eternal, immovable prime mover and an eternal prime movable, uniformly moved by the prime mover. But by the motion it in turn imparts the prime movable is the immediate cause of the rotations in nature's course, where motion alternates with rest, and generation with corruption.
e) The kind of motion caused by the prime mover.11 -
We have seen how Aristotle conceives the arrangement of essential movers and movables in the universe. But we have not yet explained what kind of motion it must be that the prime mover imparts to the prime movable. Aristotle resolves this point in three steps.
First, he declares the primacy of local motion. Growth, he observes, presupposes alteration: food has to be altered before it can be assimilated. But alteration depends on the active and passive elements being brought in contact, and this involves local motion, which is therefore primary.12
Secondly, not all local motion is the same. It may be circular or rectilinear, or a combination of the two. Only circular motion can be infinite, one, and continuous. Rectilinear motion cannot be infinite, because this would imply infinite magnitude, an impossibility; it cannot be continuous, because it necessarily involves renewal in the opposite direction. Aristotle's discussion of these propositions is not only detailed but also highly complex.13
Thirdly, Aristotle contends that circular motion is the primary local motion, hence primary to all motion. Circular translations, he argues, are simpler and more perfect than rectilinear displacements. Continuous and uniform, circular motion is pre-eminently suited to being the measure of all other motions.14
Which, in reality, is this circular, uniform, and eternal motion of which Aristotle speaks? As readily surmised, it is the motion of the first heaven or first sphere, and this sphere in consequence becomes the first movable. Thus, what emerges as a deductive or logical necessity is also found a fabric of reality. If, that is, the primacy of circular motion is argued deductively, the final deduction ends where, true to Aristotle, it originally began - in experience.