1 · Elsewhere we have investigated the movement of animals after their [698a1] various kinds, the differences between them, and the causes of their particular characters (for some animals fly, some swim, some walk, others move in various other ways); there remains an investigation of the common cause of any sort of [5] animal movement whatsoever.
Now we have already determined (when we were discussing whether eternal [10] motion exists or not, and what it is, if it does exist) that the origin of other motions is that which moves itself, and that the origin of this is the immovable, and that the prime mover must of necessity be immovable. And we must grasp this not only generally in theory, but also by reference to individuals in the world of sense; for with these in view we seek general theories, and with these we believe that general theories ought to harmonize. Now in the world of sense too it is plainly impossible [15] for movement to be initiated if there is nothing at rest, and before all else in our present subject—animal life. For if one of the parts of an animal be moved, another must be at rest, and this is the purpose of their joints; animals use joints like a centre, and the whole member, in which the joint is, becomes both one and two, both straight and bent, changing potentially and actually by reason of the joint. And [20] when it is bending and being moved one of the points in the joint is moved and one is at rest, just as if on a diameter AD were at rest, and B were moved, and AC were generated. However, in the geometrical illustration, the centre is held to be altogether indivisible (for in mathematics the motion they speak of is a fiction, no [25] mathematical entity being really moved), whereas in the case of joints the centres become potentially and actually now one, now divided. But still the origin of movement, qua origin, always remains at rest when the lower part of a limb is [698b1] moved; for example, the elbow joint, when the forearm is moved, and the shoulder, when the whole arm; the knee when the tibia is moved, and the hip when the whole leg. Accordingly it is plain that each animal as a whole must have within itself a point at rest, whence will be the origin of that which is moved, and supporting itself [5] upon which it will be moved both as a complete whole and in its members.
2 · But the point of rest in the animal is still quite ineffectual unless there is something outside it which is absolutely at rest and immovable. Now it is worth [10] while to pause and consider what has been said; for it involves a speculation which extends beyond animals even to the motion and march of the universe. For just as there must be something immovable within the animal, if it is to be moved, so even more must there be without it something immovable, by supporting itself upon [15] which that which is moved moves. For were that something always to give way (as it does for tortoises walking on mud or persons walking in sand) advance would be impossible, and neither would there be any walking unless the ground were to remain still, nor any flying or swimming were not the air and the sea to resist. And this which resists must needs be different from what is moved, the whole of it from [20] the whole of that, and what is thus immovable must be no part of what is moved; otherwise there will be no movement. Evidence of this lies in the problem why it is that a man easily moves a boat from outside, if he push with a pole, putting it against the mast or some other part, but if he tried to do this when in the boat itself [25] he would never move it, no not even Tityus himself nor Boreas blowing from inside the ship, if he really were blowing in the way painters represent him; for they paint [669a1] him sending the breath out from himself. For whether one blew gently or so stoutly as to make a very great wind, and whether what were thrown or pushed were breath or something else, it is necessary in the first place to be supported upon one of one’s own members which is at rest and so to push, and in the second place for this [5] member, either itself, or that of which it is a part, to remain at rest, fixing itself against something external to itself. Now the man who is himself in the boat, if he pushes, fixing himself against the boat, does not move the boat, because what he pushes against must remain at rest. Now what he is trying to move, and what he is [10] fixing himself against is in his case the same. If, however, he pushes or pulls from outside he does move it; for the ground is no part of the boat.
3 · Here we may ask the question whether if something moves the whole heavens this mover must be immovable, and moreover be no part of the heavens, nor in the heavens. For either it is moved itself and moves the heavens, in which case it [15] must touch something immovable in order to cause movement, and then this is no part of that which cause movement; or if the mover is from the first immovable it will equally be no part of that which is moved. In this point at least they argue correctly who say that as the sphere is carried round in a circle no single part remains still; for then either the whole would necessarily stand still or its continuity [20] be torn asunder; but they argue less well in supposing that the poles have a certain power, though they have no magnitude, but are merely termini or points. For besides the fact that no such things have any substantial existence it is impossible for a single movement to be initiated by what is twofold; and yet they make the poles [25] two. From a review of these difficulties we may conclude that there is something so related to the whole of nature, as the earth is to animals and things moved by them.
And the mythologists with their fable of Atlas setting his feet upon the earth appear to have based the fable upon intelligent grounds. They make Atlas a kind of diameter twirling the heavens about the poles. Now as the earth remains still this would be reasonable enough, but their theory involves them in the position that the [30] earth is no part of the universe. And further the force of that which initiates movement must be made equal to the force of that which remains at rest. For there is a definite quantity of force or power by dint of which that which remains at rest does so, just as there is of force by dint of which that which initiates movement does [35] so; and as there is a necessary proportion between contrary motions, so there is between states of rest. Now equal forces are unaffected by one another, but are overcome by a superiority of force. And so Atlas, or whatever similar power initiates [699b1] movement from within, must exert no more force than will exactly balance the stability of the earth—otherwise the earth will be moved out of her place in the centre of things. For as the pusher pushes so is the pushed pushed, and with equal [5] force. But that which initiates movement is to begin with at rest, so that its force is greater, rather than equal and like to the stability. And similarly also than the stability of what is moved but does not initiate movement. Therefore the power of the earth in its immobility will have to be as great as that of the whole heavens, and of that which moves the heavens. But if that is impossible, it follows that the [10] heavens cannot be moved by anything of this kind inside them.
4 · There is a difficulty about the motions of the parts of the heavens which, as akin to what has gone before, may be considered next. For if one could overcome by power of motion the immobility of the earth he would clearly move it away from [15] the centre. And it is plain that the force from which this power would originate will not be infinite; for the earth is not infinite and therefore its weight is not. Now things are called impossible in several ways; for when we say it is impossible to see a sound, and when we say it is impossible to see the men in the moon, we use the word in different ways: the former is of necessity, the latter, though their nature is to be [20] seen, will not actually be seen by us. Now we suppose that the heavens are of necessity impossible to destroy and to dissolve, whereas the result of the present argument would be to do away with this necessity. For it is natural and possible for a motion to exist greater than that by dint of which the earth is at rest, or than that by dint of which fire and the upper body are moved. If then there are superior [25] motions, these will be dissolved by one another; and if there actually are not, but might possibly be (for they cannot be infinite because not even body can be infinite), there is a possibility of the heavens being dissolved. For what is to prevent this coming to pass, unless it be impossible? And it is not impossible unless the opposite [30] is necessary. This difficulty, however, we will discuss elsewhere.1
Must there be something immovable and at rest outside of what is moved, and no part of it, or not? And must this necessarily be so also in the case of the universe? Perhaps it would be thought strange were the origin of movement inside. And to those who so conceive it the words of Homer2 would appear to have been well [35] spoken: ‘Nay, ye would not pull Zeus, highest of all, from heaven to the plain, no not even if ye toiled right hard; come, all ye gods and goddesses! Set hands to the chain’. [700a1] For that which is entirely immovable cannot possibly be moved by anything. And herein lies the solution of the difficulty stated just now, the possibility or [5] impossibility of dissolving the system of the heavens, in that it depends from an origin which is immovable.
Now in the animal world there must be not only an immovable without, but also within those things which move in place, and initiate their own movement. For one part of an animal must be moved, and another be at rest, and against this the [10] part which is moved will support itself and be moved; for example, if it moves one of its parts; for one part supports itself against another as though it were at rest.
But about things without life which are moved one might ask the question whether all contain in themselves both that which is at rest and that which initiates [15] movement, and whether they also, for instance fire, earth, or any other inanimate thing, must support themselves against something outside which is at rest. Or is this impossible and must it not be looked for rather in those primary causes by which they are set in motion? For all things without life are moved by something other, and the origin of all things so moved are things which move themselves. And out of these we have spoken about animals (for they must all have in themselves that which is at rest, and without them that against which they are supported); but [20] whether there is some higher and prime mover is not clear, and an origin of that kind involves a different discussion. Animals at any rate which move themselves are all moved supporting themselves on what is outside them, even when they breathe in and out; for there is no essential difference between casting a great and a small weight, and this is what men do when they spit and cough and when they breathe in [25] and breathe out.
5 · But is it only in that which moves itself in place that there must be a point at rest, or does this hold also of that which causes its own qualitative changes, and its own growth? Now the question of original generation and decay is different; for [30] if there is, as we hold, a primary movement, this would be the cause of generation and decay, and probably of all the secondary movements too. And as in the universe, so in the animal world this is the primary movement, when the creature attains maturity; and therefore it is the cause of growth, when the creature becomes the cause of its own growth and the cause too of alteration. Otherwise, the point at rest is not necessary. However, the earliest growth and alteration in the living creature [700b1] arise through another and by other channels, nor can anything possibly be the cause of its own generation and decay; for the mover must exist before the moved, the begetter before the begotten, and nothing is prior to itself.
6 · Now whether the soul is moved or not, and how it is moved if it be moved, [5] has been stated before in our treatise concerning it.3 And since all inanimate things are moved by some other thing—and the manner of the movement of the first and eternally moved, and how the first mover moves it, has been determined before in our work on first philosophy,4 it remains to inquire how the soul moves the body, and [10] what is the origin of movement in a living creature. For, if we except the movement of the universe, things with life are the causes of the movement of all else, that is of all that are not moved by one another by mutual impact. And so all their motions have a limit, inasmuch as the movements of things with life have such. For all living things both move and are moved for the sake of something, so that this is the limit of [15] all their movement—that for the sake of which. Now we see that the living creature is moved by intellect, imagination, purpose, wish, and appetite. And all these are reducible to thought and desire. For both imagination and sensation are on common ground with thought, since all three are faculties of discrimination though differing [20] according to distinctions stated elsewhere. Wish, however, impulse, and appetite, are all three forms of desire, while purpose belongs both to intellect and to desire. Therefore the object of desire or of intellect first initiates movement—not every object of intellect, but only the end in the domain of conduct. Accordingly it is goods [25] of this sort that initiate movement, not everything fine. For it initiates movement only so far as something else is for its sake, or so far as it is the end of that which is for the sake of something else. And we must suppose that a seeming good may take the room of actual good, and so may the pleasant, which is itself a seeming good. From these considerations it is clear that in one regard that which is eternally [30] moved by the eternal mover is moved in the same way as every living creature, in another regard differently, and so while it is moved eternally, the movement of living creatures has a limit. Now the eternally fine, and the truly and primarily good (which is not at one time good, at another time not good), is too divine and precious to be relative to anything else. The prime mover then moves, itself being unmoved, whereas desire and its faculty are moved and so move. But it is not necessary for the [701a1] last in the chain of things moved to move something else; wherefore it is plainly reasonable that motion in place should be the last of the movements in things that come into being; for the living creature is moved and goes forward by reason of desire or purpose, when some alteration has been set going on the occasion of [5] sensation or imagination.
7 · But how is it that thought is sometimes followed by action, sometimes not; sometimes by movement, sometimes not? What happens seems parallel to the case of thinking and inferring about the immovable objects. There the end is the truth seen (for, when one thinks the two propositions, one thinks and puts together [10] the conclusion), but here the two propositions result in a conclusion which is an action—for example, whenever one thinks that every man ought to walk, and that one is a man oneself, straightaway one walks; or that, in this case, no man should walk, one is a man: straightaway one remains at rest. And one so acts in the two [15] cases provided that there is nothing to compel or to prevent. Again, I ought to create a good, a house is good: straightaway he makes a house. I need a covering, a coat is a covering: I need a coat. What I need I ought to make, I need a coat: I make a coat. [20] And the conclusion ‘I must make a coat’ is an action. And the action goes back to a starting-point. If there is to be a coat, there must first be this, and if this then this—and straightaway he does this. Now that the action is the conclusion is clear. But the premisses of action are of two kinds, of the good and of the possible.
[25] And as sometimes happens in dialectical questioning, so here the intellect does not stop and consider at all the one proposition, the obvious one; for example if walking is good for man, one does not dwell upon the proposition ‘I am a man’. And so what we do without reflection, we do quickly. For when a man is actually using [30] perception or imagination or thought in relation to that for the sake of which, what he desires he does at once. For the actualizing of desire is a substitute for inquiry or thinking. I want to drink, says appetite; this is drink, says sense or imagination or thought: straightaway I drink. In this way living creatures are impelled to move and [35] to act, and desire is the last cause of movement, and desire arises through perception or through imagination and thought. And things that desire to act make and act [701a] sometimes from appetite or impulse and sometimes from wish.
The movements of animals may be compared with those of automatic puppets, which are set going on the occasion of a tiny movement (the strings are released, and the pegs strike against one another); or with the toy wagon (for the child mounts on it and moves it straight forward, and yet it is moved in a circle owing to [5] its wheels being of unequal diameter—the smaller acts like a centre on the same principle as the cylinders). Animals have parts of a similar kind, their organs, the sinewy tendons to wit and the bones; the bones are like the pegs and the iron; the tendons are like the strings; for when these are slackened or released movement [10] begins. However, in the puppets and the toy wagon there is no change of quality, since if the inner wheels became smaller and greater by turns there would be the same circular movement set up. In an animal the same part has the power of becoming now larger and now smaller, and changing its form, as the parts increase [15] by warmth and again contract by cold and change their quality. This change of quality is caused by imaginations and sensations and by ideas. Sensations are obviously a form of change of quality, and imagination and thinking have the same [20] power as the objects. For in a measure the form conceived be it of hot or cold5 or pleasant or fearful is like what the actual objects would be, and so we shudder and are frightened merely by thinking. Now all these affections are actually changes of quality, and with those changes some parts of the body enlarge, others grow smaller. [25] And it is not hard to see that a small change occurring at the centre makes great and numerous changes at the circumference, just as by shifting the rudder a hair’s breadth you get a wide deviation at the prow. And further, when by reason of heat or cold or some kindred affection a change is set up in the region of the heart, even in [30] an imperceptibly small part of the heart, it produces a vast difference in the body—blushing, let us say, or turning white, and tremblings and shivers and their opposites.
8 · But to return, the object we pursue or avoid in the field of action is, as has been explained, the origin of movement, and upon the thought and imagination of [35] this there necessarily follows a heating or chilling. For what is painful we avoid, what is pleasing we pursue, and anything painful or pleasing is generally speaking [702a1] accompanied by a chilling and heating (but we do not notice this when it happens in a small part). One may see this by considering the affections. Blind courage and panic fears, erotic motions, and the rest of the corporeal affections, pleasant and painful, are all accompanied by heating or chilling, some in a particular member, others in the body generally. So, memories and anticipations, using things of this [5] kind as likenesses, are now more and now less causes of the same changes of temperature. And so we see the reason of nature’s handiwork in the inward parts, and in the centres of movement of the organic members; they change from solid to moist, and from moist to solid, from soft to hard and vice versa. And so when these [10] are affected in this way, and when besides the passive and active have the constitution we have many times described, as often as it comes to pass that one is active and the other passive, and neither of them falls short of the elements of its account, straightaway one acts and the other responds. And on this account [15] thinking that one ought to go and going are virtually simultaneous, unless there be something else to hinder action. The organic parts are suitably prepared by the affections, these again by desire, and desire by imagination. Imagination in its turn depends either upon thinking or upon sense-perception. And the simultaneity and [20] speed are due to the natural correspondence of the active and passive.
However, that which first moves the animal organism must be in a definite origin. Now we have said that a joint is the origin of one part of a limb, the end of another. And so nature employs it sometimes as one, sometimes as two. When movement arises from a joint, one of the extreme points must remain at rest, and the [25] other be moved (for as we explained above the mover must support itself against a point at rest); accordingly, in the case of the elbow-joint, the last point of the forearm is moved but does not move anything, while, in the flexion, one point of the elbow, which lies in the whole forearm that is being moved, is moved, but there must also be a point which is unmoved, and this is our meaning when we speak of a point [30] which is in potency one, but which becomes two in actual exercise. Now if the forearm were the living animal, somewhere in its elbow-joint would be the movement-imparting origin of the soul.
Since, however, it is possible for a lifeless thing to be so related to the hand as the forearm is to the upper (for example, when a man moves a stick in his hand), it is evident that the soul could not lie in either of the two extreme points, neither, that is, [35] in the last point of what is moved, nor in the other origin. For the stick too has an end point and an origin by reference to the hand. Accordingly, for this reason, if the [702b1] moving origin which derives from the soul is not in the stick, then it is not in the hand either; for a precisely similar relation obtains between the extremity of the hand and the wrist, and between the wrist and the elbow. In this matter it makes no difference whether the part is naturally connected to the body or not; the stick may [5] be looked at as a detached part-of the whole. It follows then of necessity that it cannot lie in any origin which is the end of another member, even though there may lie another part outside the one in question. For example, relatively to the end point of the stick the hand is the origin, but the origin of the hand’s movement is in the [10] wrist. And so if it is not even in the hand, because there is still something higher up, neither is the origin here; for once more if the elbow is at rest the whole part below it can be moved as a continuous whole.
9 · Now since the left and the right sides are symmetrical, and these opposites are moved simultaneously, it cannot be that the left is moved by the right [15] remaining stationary, nor vice versa; the origin must always be in what lies above both. Therefore, the origin of the moving soul must be in the middle; for of both extremes the middle is the limiting point; and this is similarly related to the movements from above and below—e.g. from the head—and to those which spring from the spinal column, in creatures that have a spinal column.
[20] And this is a reasonable arrangement. For the sensorium is in our opinion in the centre too; and so, if the region of the origin is altered through sense-perception and thus changes, the adjacent parts change with it and they too are extended or contracted, and in this way the movement of the creature necessarily follows. And [25] the middle of the body must needs be in potency one but in actuality more than one; for the limbs are moved simultaneously from the origin, and when one is at rest the other is moved. For example, in the line ABC, B is moved, and A is the mover. There [30] must, however, be a point at rest if one is to move, the other to be moved. A then being one in potency must be two in actuality, and so be a magnitude not a point. Again, C may be moved simultaneously with B. Both the origin then in A must move and be moved, and so there must be something other than them which moves [35] but is not moved. For otherwise, when the movement begins, the extremes, i.e. the origin, in A would rest upon one another, like two men putting themselves back to [703a1] back and so moving their legs. There must then be some one thing which moves both. This something is the soul, distinct from the magnitude just described and yet located therein.
10 · Although from the point of view of the account which gives the cause of [5] movement desire is the middle, and desire moves being moved, still in the animated body there must be some body of this kind. Now that which is moved, but whose nature is not to initiate movement, is capable of being passive to an external power, while that which initiates movement must needs possess a kind of force and power. Now it is clear that animals do both possess connatural spirit and derive force from [10] this. (How this connatural spirit is maintained in the body is explained in other passages of our works.) And this spirit appears to stand to the soul-origin in a relation analogous to that between the point in a joint which moves being moved and the unmoved. Now since this origin is for some animals in the heart, in the rest in a [15] part analogous with the heart, for this reason the connatural spirit is clearly there too. (The question whether the spirit remains always the same or constantly changes must be discussed elsewhere; for the same question arises about the rest of the parts of the body.) At all events we see that it is well disposed to excite movement and to exert force. Now the functions of movement are thrusting and pulling. Accordingly, the organ of movement must be capable of expanding and [20] contracting; and this is precisely the characteristic of spirit. It contracts and expands without constraint, and so is able to pull and to thrust from one and the same cause, exhibiting weight compared with the fiery element, and lightness by comparison with the opposites of fire. Now that which is to initiate movement [25] without alteration must be of the kind described; for the natural bodies prevail over one another by dint of predominance; the light is overcome and kept down by the heavier, and the heavy kept up by the lighter.
We have now explained what the part is which is moved when the soul originates movement, and what is the reason for this. And the animal organism must be conceived after the similitude of a well-governed commonwealth. When [30] order is once established in a city there is no more need of a separate monarch to preside over each several task. The individuals each play their assigned part as it is ordered, and one thing follows another because of habit. So in animals the same thing happens because of nature, each part naturally doing its own work as nature [35] has composed it. There is no need then of a soul in each part, but it resides in a kind of origin of the body, and the remaining parts live by being naturally connected, and [703b1] play their parts because of their nature.
11 · So much then for the voluntary movements of animal bodies, and the reasons for them. These bodies, however, display in certain members involuntary [5] movements too, but most often non-voluntary movements. By involuntary I mean motions of the heart and of the penis; for often upon an image arising and without express mandate of the intellect these parts are moved. By non-voluntary I mean sleep and waking and respiration, and other similar movements. For neither imagination nor desire is properly mistress of any of these. But since the animal [10] body must undergo natural changes of quality, and when the parts are so altered some must increase and others decrease, so that the body must straightaway be moved and change with the changes that nature makes dependent upon one another (the causes of the movements are heatings and chillings, both those coming from [15] outside the body, and those taking place naturally within it)—so the movements which occur in spite of reason in the aforesaid parts occur when a change of quality supervenes. For thinking and imagination, as we said above, produce that which brings about the affections, since they produce the forms which bring them about. [20] And the parts aforesaid display this motion more conspicuously than the rest, because each is in a sense a separate animal, the reason being that each contains vital moisture.6 In the case of the heart the cause is plain; for it contains the origins of the senses; while an indication that the generative organ too is vital is that there [25] flows from it the seminal potency, itself a kind of living creature. Again, it is a reasonable arrangement that the movements arise in the origin upon movements in the parts, and in the parts upon movements in the origin, and so reach one another. [30] Conceive A to be the origin. The movements then arrive at the origin from each letter in the diagram we have drawn, and back again from the origin which is moved and changes, (for it is potentially multiple) the movement of B goes to B, that of C to C, the movement of both to both; but from B to C the movements flow by dint of [35] going from B to A as to an origin, and then from A to C as from an origin. And a movement contrary to reason sometimes does and sometimes does not arise in the parts on the occasion of the same thoughts; the reason is that sometimes the matter [704a1] which is passive is there in sufficient quantity and of the right quality and sometimes not.
And so we have finished our account of the reasons for the parts of each kind of [704b1] animal, of the soul, and further of sense-perception, of sleep, of memory, and of movement in general; it remains to speak of generation.