Physics VIII is an argument for the eternal motion as well as for the necessary existence of such a being as the first unmoved mover that must lie at the foundation of all motion, if nature is to be defined as a principle of motion. In this paper, I will attempt to explicate exegetically what such a principle of motion is for Aristotle, and hope to clarify what is meant by the prime unmoved mover insofar as it deals with the realm of physics. In doing so, I will try to outline the main argument of Physics VIII.
1: Preliminary Remarks
In structuring the argument, Aristotle follows what has been outlined in Posterior Analytics II.1, namely, the investigation of the four things – the fact (that it is), the reason why (why it is), whether it is (if it is) and what it is. But because to seek the fact that the moon is eclipsed is the same as to seek whether it is eclipsed, it suffices to ask if it is eclipsed to account for both things. In the same manner, to ask what it is and why it is are one and the same, since asking what an eclipse is is the same as asking why the moon is eclipsed – privation of light from the moon by the earth’s screening, i.e. the light leaves the earth when the earth screens the moon. In this way, the original four questions we seek to investigate are reduced to two: whether it is (ει εστι) and what it is (τι εστι). Physics VIII. 1 is hence dedicated to answering the first of the two questions, namely, whether motion is. In asking whether/what motion is, Aristotle reviews commonly held opinions (δοξαι) by his predecessors as possibilities for whether motion is. Although the existence of motion is asserted by many who speak of nature, neither Anaxagoras, who says all things were together until Nous introduced motion and separated them, nor Empedocles, who makes motion alternate with rest, nor Democritus, who postulates that there always was motion and there always will be, fails to give a full account of why motion started when it started and how. For this reason, Aristotle thinks that none of them did a sufficient job at explaining the phenomenon, that is, they did not investigate the causal stage of the explanation (τι εστι).
In establishing that motion always existed, it also follows that time as well always existed, because motion is unthinkable without time, and vice versa. This is intuitive since time cannot be thought of apart from the now, but now is nothing but the uniting point of the beginning of future time and the end of past time, so it follows that there must always be time and consequently motion, for “the extremity of the last period of time that we take must be found in some now, since in time we can take nothing but nows.” It is then necessary to conclude that time as well as motion have always existed and will always exist. Aristotle’s task for the following chapters in Physics VIII is to explain why it is the case that motion and time have always existed and how it is the case: that is to say, to elucidate what he has already established, namely, that motion is eternal. In doing so, he will come to the conclusion that if there is to be an eternal motion, it is necessary that the first thing that imparts motion must be without magnitude and without parts, in addition to it being absolutely unmoved. Let us, now, unfold his argument so as to better understand what he means by the eternal motion caused by this unmoved mover.
Section 2: Physics VIII 6
Aristotle has discussed that nature is a principle of motion and observes that everything that is in motion must be moved by something. Further, there must be three things: the moved, the mover and the instrument of motion. Conversely, he arrives at the tentative conclusion that “[s]ince there must always be motion without intermission, there must necessarily be something eternal… that first imparts motion, and this first mover must be unmoved.” This first mover that is absolutely unmoved, i.e. unmoved not even accidentally, he calls the prime unmoved mover.
The prime unmoved mover is introduced in Physics VIII 6 only to an extent necessary to talk about the subject of this treatise, which is eternal motion. To understand why Aristotle does not discuss about it in one chapter or deal with it in the consecutive chapters, instead of dividing it up into two chapters separately located, we must first see what is discussed in the chapters 7 through 9 and in what way they are necessarily connected with the chapter 6. In doing so, I will first show that it was necessary for Aristotle to briefly introduce the prime unmoved mover if he wanted to talk about the existence of eternal motion, which is what was to be demonstrated in Physics VIII, and then offer an explanation as to why he felt he needed yet another chapter on the prime unmoved mover even though he has demonstrated the necessity of eternal motion in the preceding chapters already.
In Physics VIII 6, he raises three arguments that since there must always be motion without intermission, there must be something that which imparts motion, and its motion must be one and continuous. The subsequent chapters are, thus, concerned with what this eternal motion could be, and how it could come about. He has established in the chapter 5 that since everything that is in motion is necessarily moved by something else or by itself, and since this cannot go on ad infinitum, there must be a point at which this recession ends. And this thing that which moves with itself “must be composed of something that is unmoved but imparts motion and also of something that is moved but does not necessarily impart motion,” because “it is impossible that that which moves itself should in its entirety move itself.” In an animate thing, there always must be a part of it that is unmoved mover and a part that is moved, so that one part that is unmoved can move that which is moved. But if this unmoved mover itself contains a further part of it with which it imparts motion, then the first mover is this part of the unmoved mover and not the whole of it, and it goes on ad infinitum. Hence it is necessary that, in the whole of the thing, there is always both a part that which imparts motion without being itself moved and a part that which is moved. Therefore, Aristotle concludes at the end of the chapter 5 that “in all cases of things being in motion, that which primarily imparts motion is unmoved.” This, then, is an account for how self-motion starts in animals, but even this motion is not strictly speaking generated from within the animals, as “here the motion is caused by the environment and by many things that enter into the animal,” such as nourishment. So in the case of animals, it is clear that the motion does not originate from them, but “it is something else that moves them.” It is also the case that although self-motion in animals is caused in the way mentioned above, the unmoved mover in animals do move accidentally, since when the animal moves, the unmoved part in the body of the animal also moves. But that which moves in this way, i.e. accidentally, cannot produce an eternal continuous motion. Hence, we are now looking for the unmoved mover that is not moved even accidentally. Such a mover must exist to account for the undying and unceasing motion in the world, which remains self-contained, as the principle of nature.
Naturally, his next step is to ask if this unmoved mover is one and eternal. As has been established that motion is eternal elsewhere, he reasons that this first mover which imparts eternal motion too must be eternal, since nothing finite can exert infinite power so as to cause an eternal motion. Further, that there is only one first mover rather than many is shown by the fact that the consequences of either assumptions are the same, and in such a case, it is sufficient to suppose only one mover that is the first of unmoved things and is the principle of everything else.
This eternal first mover, then, must cause an eternal motion that is the subject of the physics. The discussion of the prime unmoved mover, therefore, is necessary to explain the eternal motion. Because we observe motion (effect) to be eternal, there must be a cause that is responsible for its effect. This necessity of such a cause is argued from the effect to the extent it is needed, i.e. the cause needs to be mentioned insofar as it explains the origin of the effect seen. And because it is motion, and not the prime unmoved mover, that is the subject of the investigation in physics, Aristotle is obligated to leave the discussion of the first mover in pursuit of the aim of the treatise. Consequently, having posited that there must be the cause of all motion, which is inferred from the effect we observe, he reverses his argument back from the cause to the effect. That is, if there is to be such a cause, i.e. the prime unmoved mover, then there has to follow such and such an effect. Since this prime unmoved mover is one and eternal, the motion it produces too must be one and eternal. What could such a motion be? The chapters 7 through 9 ask about this very subject that is at the heart of the project in Physics, and illuminate what kind of motions is said to be one and eternal.
Section 3: Physics VIII 7-9
As has been discussed elsewhere, there can only be four types of motion: generation and corruption, growth and decay, locomotion, and alteration. The first two of these are concerned with quantitative change, alteration is with qualitative change, and locomotion is a motion in respect of place. Since neither quantitative nor qualitative change can take place without locomotion, this must be the primary type of motion and therefore prior in time and in being to the other types of motion. But we have yet to deal with what kinds of locomotion there are, to which we immediately find that there are two kinds only: rectilinear motion or circular motion. The former of the two cannot be the primary motion that is both one and continuous, since a thing in a finite rectilinear motion must at some point come to a halt to turn back, and “that which turns back in a straight line undergoes two contrary locomotions.” It could not be the case either that a thing is in an infinite rectilinear motion, since it being rectilinear, it must have a beginning at some point, and that which has a beginning must have an end also. That such motion cannot be one and eternal is obvious, since it must come to a stand at some point (hence, rendering it not-continuous) and it undergoes contrary motions (two motions are specifically different, and hence cannot be one). As for the circular motion, it satisfies both requirements, that is, a thing in a circular motion will not come to a stand but can continue to move without ever coming to a stop. Moreover, there are no contrary motions (i.e. motions that are specifically different) in a circular motion, for “that which is in motion from A will in virtue of the same direction of energy be simultaneously in motion to A.” From what has been said, it is established that only circular locomotion can be one and eternal, and only this kind of motion is perfect.
Only now is he able to arrive at the conclusion that circular locomotion is the eternal motion, which is the proper subject of physics. He started out by observing the commonsense fact that there are things that are always in motion, always at rest, and sometimes in motion and sometimes at rest. Cyclical motion of the four elements accounts for our observation that generation and corruption, etc., occur at one time and do not at another time. From this, Aristotle infers back to the necessary existence of the prime unmoved mover that imparts motion continuously. But such motion must necessarily be eternal, and things moved (the subject; material cause) by such a mover (efficient cause) must be moved eternally. That which is eternally moved is eternal circular locomotion. Having explained the phenomenon both from effect to cause and from cause to effect, his treatise on physics is complete, except for the fact that he has not said much about what this prime unmoved mover is. The prime unmoved mover is mentioned in the chapter 6 only as an efficient cause responsible for the eternal circular motion, but we get no picture whatsoever of what type of being this prime unmoved mover is. This is why Aristotle felt necessary to talk about the prime unmoved mover in the last chapter of Physics, and to discuss about it insofar as it remains within the realm of physics to give a more precise description of what this prime unmoved mover is.
Section 4: Physics VIII 10
The chapter 10, then, is concerned with a more precise account of the prime unmoved mover as an efficient cause responsible for such motion as eternal circular motion. Because of this reason, the precise nature of what the prime unmoved mover is does not fall within the subject of physics. That is, for what purpose it causes eternal motion is not the scope of this treatise, but it is sufficient here to note that it does cause eternal motion. Some things, however, about this unmoved mover still fall within the subject of our enquiry in physics: namely, whether it has magnitude or what kind of force it has, i.e. finite or infinite? At the end of the chapter 6, it was neither necessary nor possible for Aristotle to ask these questions about the prime unmoved mover, for he was not concerned with what it is but only with the prime unmoved mover as an efficient cause to introduce the subject, which revealed to be the eternal circular locomotion. Insofar as that was his aim, more discussions than necessary into the details of the prime unmoved mover would be distracting to the argument. But now that Aristotle has discussed fully about the eternal motion as the material cause of all motions we see in general physics, he is in a position to say a few more things about the features of the prime unmoved mover.
Here Aristotle argues that this prime unmoved mover must be 1) infinite and 2) without parts and without magnitude. That it is infinite is arrived at by two premises he has established in the works of Physics; namely, that nothing finite can cause motion during an infinite time, and that the prime unmoved mover must have an infinite force if it were to exert continuous motion eternally. Because when a thing moves, it loses some force upon contact with another, although this mover “causes something else consecutive with it to be in motion,” this motion “ceases when the motive force produced in one member of the consecutive series is at each stage less, and it finally ceases when one member no longer causes the next member to be a mover but only causes it to be in motion,” leading up to the eventual cessation of the whole motion. So if this prime unmoved mover has finite force, it will exhaust itself at some point, no longer capable of exerting motion, which is contrary to the phenomenon. It then must have an infinite force. But a finite body cannot have an infinite force, hence it follows that this prime unmoved mover is infinite. If it is infinite, then, it must possess infinite magnitude, but Aristotle has shown that it is impossible to have an infinite magnitude. But a difficulty arises, since we have said that no finite body can have infinite force, then it must follow that this prime unmoved mover must possess an infinite magnitude, which we have said is also impossible. So here, we are faced with a question: is the prime unmoved mover a magnitude? For if it is, then it has to be either infinite or finite, but infinite magnitude does not exist and finite magnitude cannot have an infinite force, which is requisite of the eternal mover, because otherwise motion would eventually cease, as we have said. It then is either a magnitude or not magnitude. But since neither infinite nor finite magnitude is possible, it must be that it has no magnitude. Furthermore, what is without magnitude is necessarily without parts, since whatever has parts is moved by its part, and is in motion. But the prime unmoved mover cannot be in motion even accidentally. Hence, Aristotle argues, it follows that this prime unmoved mover is of necessity without magnitude or parts, and is infinite.
 Ibid., Posterior Analytics II. 1-2, 89b23-90a34.
 Ibid., Physics VIII.1.
 Ibid. italics mine.
 Physics III 1.
 Aristotle, Physics VIII 5, 256b14-16. Aristotle continues to say that the moved must be in motion, but need not move anything else; the instrument of motion must both move something else and be itself in motion; the mover, which causes motion in such a manner that it is not merely the instrument of motion but also must be unmoved.
 Aristotle, Physics VIII 6, 258b10-12.
 VIII 5 mentions about the first unmoved mover as a hypothesis, but Aristotle begins to discuss about it extensively only in chapter 6.
 A more precise description of what the prime unmoved mover is is given at the very end of the treatise in the chapter 10.
 VIII 6, 258b10-12. See also 259a15-19 where he says that motion must be continuous “because what is always is continuous, whereas what is in succession is not continuous. But further, if motion is continuous, it is one… since in the event of a thing’s being moved now by one thing and now by another the whole motion will not be continuous but successive.
 See VIII 5, where Aristotle uses an analogy of a person moving something with a stick. “Every mover moves something and moves it with something, either with itself or with something else,” because that which is in motion, if it has no motive agency of its own, must be moved by something else other than by itself, and that too must be moved by something else, and so on, until we arrive at something that which imparts motion with itself. 256a22-256b3.
 See VIII 5, 258a8-9, 257b2. For a detailed explanation for why it is impossible, see also De Motu I 1, 669a1-669a6, “…it is necessary in the first place to be supported one of one’s own members which is at rest and so to push, and in the second place for this member, either itself, or that of which it is a part, to remain at rest, fixing itself against something external to it” in order to move.
 259b13-15, “when it is being digested animals sleep, and when it is being distributed they awake and move themselves, the first principle of this being thus originally derived from outside,” which clearly shows that “animals are not always in continuous motion by their own agency.”
 VIII 6, 259b22-28.
 See VIII 1.
 VIII 6, 259a9-13.
 See VIII 6, 259b32-33, “if there is always something of this nature, a mover that is itself unmoved and eternal, then that which is first moved by it must also be eternal.” Also 260a15-19, “while some things are moved by an eternal unmoved mover and are therefore always in motion, other things are moved by something that is in motion and changing, so that they too must change. But the unmoved mover, as has been said, since it remains simple and unvarying and in the same state, will cause motion that is one and simple.”
 Physics VII 2.
 See 260a27-28. “Now of the three kinds of motion that there are – motion in respect of magnitude [quantitative], motion in respect of affection [qualitative], and motion in respect of place – it is this last, which we call locomotion, that must be primary,” since “no other motion can be continuous except locomotion.” See also 260b29-30, “locomotion must be primary in time; for this is the only motion possible for eternal things.” And also 261a19, “…this motion [locomotion] must be prior to all others in respect of being.”
 The impossibility of an infinite rectilinear motion is also mentioned at VIII 9, 265a17-19, “the line traversed rectilinear motion cannot be infinite; for there is no such thing as an infinite straight line; and even if there were, it would not be traversed by anything in motion,” since “it is impossible to traverse an infinite distance.” Again see 265a29-32, “[i]n rectilinear motion we have a definite beginning, end, and middle, which all have their place in it in such a way that there is a point from which that which is in motion will begin and a point at which it will end.”
 264b17-19, “there is nothing no prevent the motion being continuous and free from all intermission; for [circular] motion is motion of a thing from its place to its place, whereas rectilinear motion is motion from its place to another place.”
 264b10-12, “since [in either way it starts its motion] it is in motion to the point at which it will finally arrive.” This also because “in its locomotion it is proceeding always about a central point and not to an extreme point; and because this remains still, the whole is in a sense always at rest as well as continuously in motion.” See 265b6-8.
 264b23-24. See also VIII 9, 265a16-17, where he says “[circular] locomotion is prior [in time and in being] to rectilinear locomotion, because it is more simple and complete.”
 Aristotle’s argument style is known as a ‘progressive revelation’, in that he starts out as knowing only commonsense knowledge, and from there refutes implausible assumptions and conclusions and develops a most rational theory, finally arriving at more informed conclusions. For example, Aristotle could not have argued or known that circular locomotion is the eternal motion he wanted to prove until he has arrived at this point in his argument.
 VIII 10, 267a6-10.
 VIII 10.