Aristotle · a new plain-English translation from the original language
1| ON THE MOVEMENT OF ANIMALS As for the movement of animals — whatever belongs to each particular kind of animal, what the differentiae are, and what the causes are of the attributes that belong to each — all this has been examined elsewhere, for every kind. But we must now examine, in a general way, the common cause of being moved with any kind of motion whatsoever (for some animals move by flying, some by swimming, some by walking, and others in other such ways). That the starting point of the other motions is that which moves itself, and that the starting point of this is what is unmoved, and that the first mover must necessarily be unmoved — this has been settled earlier, at the point where we also dealt with everlasting motion: whether it exists or not, and if it exists, what it is. But this must be grasped not only by argument, but also in the particular perceptible cases, on account of which we also seek the universal accounts, and to which we think the accounts ought to apply. For it is evident, even in these particular cases, that motion is impossible unless something remains at rest — first of all within the animals themselves. For if any of the parts is in motion, something must be at rest; and this is why animals have joints. For they use their joints as a pivot, and the whole part in which the joint lies becomes both one and two, both straight and bent, changing in potentiality and in actuality on account of the joint.
2| When a limb is bending and moving, one point in the joint moves and another stays still, just as, if of a diameter the point A and the point Δ were to stay still while B moved, the line AΓ would come to be. But in that case the center is thought to be in every way indivisible (indeed, they even fabricate the notion of its being moved, as they say, in dealing with such things — for nothing belonging to the objects of mathematics is actually moved), whereas the points in the joints become, in potentiality and in actuality, at one time one and at another divisible. But at any rate the starting point on the side toward which — insofar as it is a starting point — always remains at rest while the part below it is moved: for instance, when the forearm moves, the elbow-joint remains at rest, and when the whole limb moves, the shoulder remains at rest; and of the shin, the knee, and of the whole leg, the hip. That each thing, then, must have within itself something at rest, from which the starting point of the moving part will proceed, and against which, bracing itself, it will be moved both as a whole together and part by part, is evident.
3| 2 But every state of rest within the animal itself is nonetheless without authority unless there is something outside, wholly at rest and unmoved. It is worth pausing to examine this claim; for the inquiry bears not only on animals, but also on the motion and locomotion of the universe as a whole. For just as there must be something unmoved within the animal itself, if it is to be moved, so still more must there be something outside the animal, unmoved, against which the thing that moves braces itself as it moves. For if the support always gives way — as with worms in the earth, or with creatures walking in sand — the animal makes no headway; and there will be neither walking, unless the earth stays still, nor flying nor swimming, unless the air or the sea offers resistance in return. And this must necessarily be other than the thing moved, and whole other than whole, and no part of it may be a part of the thing moved, in this sense unmoved; otherwise it will not be moved. A proof of this is the puzzle: why is it that, from outside, if someone pushes the mast of a ship — or presses against any other part — with a pole, he moves it easily, whereas if someone standing in the ship itself tries to do this, he could not move it, not even if Tityos or Boreas were blowing from inside the ship, supposing he blew in the manner painters depict him blowing —
4| for they paint him letting the wind go forth from himself. For whether one hurls the breath gently or so forcefully as to produce the greatest wind, or whether it is anything else that is hurled or pushed, it is necessary first to push by bracing against some part of oneself that is at rest, and then, in turn, for that part — either itself or that of which it happens to be a part — to remain still by being braced against something external. Now the man pushing the ship while standing in the ship and bracing himself against the ship reasonably does not move the ship, because that against which he braces himself must necessarily stay still; but it turns out for him that the very thing he moves and the thing against which he braces himself are the same. Pushing or pulling from outside, however, he does move it; for the earth is no part of the ship. One might raise the puzzle whether, if something moves the whole heaven, that thing 3 must be unmoved, and be no part of the heaven at all, nor within the heaven. For whether it moves the heaven while itself being moved, it must move it by touching something unmoved, and this must be no part of the mover;
5| or if the mover is straightaway itself unmoved, likewise it will be no part of the thing moved. And in this respect those are right who say that, the sphere being carried around in a circle, not even the smallest part of it remains at rest; for either the whole would necessarily remain at rest, or its continuity would be torn apart. But in supposing that the poles have some power, though they have no magnitude but are extremities and points, they are not right. For besides the fact that no such point has any substance at all, it is also impossible for the one motion to be moved by two things; and they make the poles two. That there is something which stands to the whole of nature as the earth stands to animals and to the things that move by means of it, one might raise as a puzzle from considerations of this sort. Those who mythically make Atlas have his feet upon the earth, might seem to have spoken the myth with some thought behind it, on the supposition that he, functioning like a diameter, turns the heaven about the poles; and this would also follow reasonably enough from the earth's staying at rest — but those who say this must necessarily also say that the earth is no part of the whole. And besides this, one must make the strength of the mover equal to that of the thing that stays at rest.
6| For there is a certain quantity of strength and power in virtue of which the thing at rest remains at rest, just as there is one in virtue of which the mover moves; and there is necessarily some proportion, just as there is among opposite motions, so too among states of rest. And things equal are unaffected by one another, but are overpowered according to the excess. And so whether it is Atlas or something else of this kind that moves the heaven from within, it needs to resist no more than the resistance which the earth in fact exerts in remaining at rest — otherwise the earth will be moved away from the middle and out of its own place. For as the pusher pushes, so the thing pushed is pushed, and equally in respect of strength. And it is what is at rest that moves in the primary sense, so that the strength of rest must be greater rather than merely similar and equal. And likewise too the strength of the thing that is moved but does not itself move something else. The power of the earth in remaining at rest will therefore need to be as great as that possessed by the whole heaven together with the thing that moves it. But if this is impossible, it is also impossible for the heaven to be moved by any such internal mover.
7| There is a certain puzzle about the motions of the parts of the heaven, which one might examine as belonging to what has been said. For if someone should exceed, in the power of the motion, the earth's being at rest, clearly he will move it from the middle. And it is clear that the strength from which this power comes is not infinite; for the earth is not infinite either, and so neither is its weight. Since "impossible" is said in several ways (for we do not say in the same way that it is impossible for sound to be seen and for the things on the moon to be seen by us; the one is impossible of necessity, the other, though naturally fitted to be seen, simply will not be seen), and we think the heaven is indestructible and cannot be dissolved of necessity, but on this argument it turns out not to be so of necessity (for it is possible and in nature's way that there be a motion greater than that by which the earth is at rest and than that by which fire and the upper body are moved); if then there are motions that exceed it, these will be dissolved by one another; and if there are not, but it is possible for there to be (for an infinite motion is not possible, since no body can be infinite either), then it would be possible for the heaven to be dissolved. For what prevents this from happening, if indeed it is not impossible? And it is not impossible, unless the opposite is necessary. But let there be another discussion about this puzzle. But is it necessary that there be something unmoved and at rest outside what is moved, being no part of it, or not? And is it necessary that this hold in this way for the universe as well? For perhaps it would seem strange if the principle of motion were internal to it. That is why it would seem, to those who suppose this, that Homer spoke well: "but you could not drag down from heaven to the plain Zeus, highest of all, not even if you toiled very hard—
8| all you gods, and all you goddesses, hang on together." For what is altogether unmoved cannot be moved by anything. This is also how the puzzle stated earlier is resolved, whether the constitution of the heaven can or cannot be dissolved, given that it is suspended from an unmoved principle. But in the case of animals, it is not only necessary that something unmoved of this sort exist, but also, within the very things that are moved with respect to place, that there be something which moves them themselves. For part of the animal must be at rest and part in motion, and the part in motion will move by pressing against the part that is at rest, as happens whenever something moves one of its parts; for the one part presses against the other as against something that stays still. About lifeless things that are moved, one might raise the puzzle whether all of them have within themselves both what is at rest and what moves, and whether these too must press against something outside that is at rest, or whether this is impossible — for instance for fire or earth or any other lifeless thing — but rather that they are moved by the first things that move them. For all lifeless things are moved by something else, and the principle of all things that are moved in this way is the things that move themselves. Of such things, what concerns animals has been stated; for all things of this kind must necessarily have within themselves what is at rest, and also something outside against which they will press. But whether there is something higher and moving in the primary sense is unclear, and that is a separate discussion, about a principle of that sort. As for the animals that are moved, all of them move by pressing against what is outside them, even in breathing in and breathing out. For it makes no difference whether one throws off a large weight or a small one — which is exactly what those do who spit, and cough, and breathe in and breathe out.
9| Is it only in the thing that moves itself with respect to place that something must remain still, or also in the thing that is altered by itself and grows? Concerning original coming-to-be and perishing there is another account; for if there is, as we say, a first motion of coming-to-be and perishing, this would be the cause, and perhaps of all the other motions as well. And just as in the universe, so too in the animal this is the first motion, once it has reached completion; so that if growth ever occurs, the animal is itself the cause of it, and likewise of alteration — but if not, this is not necessary. The first instances of growth and alteration come about through something else and by means of other things; but of coming-to-be and perishing, nothing can in any way be its own cause. For what moves must exist before what is moved, and what begets before what is begotten; and nothing is prior to itself.
10| Concerning the soul, then — whether it is moved or not, and if it is moved, how it is moved — this has been discussed earlier, in the treatises that give the definitions concerning it. And since all lifeless things are moved by something else, and it has been determined earlier, in the works on first philosophy, concerning the first thing moved and always moving, in what way it is moved and how the first mover moves it, what remains is to consider how the soul moves the body, and what the principle of the animal's motion is. For among the other things, apart from the motion of the whole, ensouled things are causes of motion — namely, all those that are not moved by one another through colliding with one another. That is also why all their motions have a limit; for so do those of ensouled things as well. For all animals both move and are moved for the sake of something, so that this — the that-for-the-sake-of-which — is, for them, the limit of all their motion. Now we observe that what moves the animal is thought, imagination, choice, wish, and appetite. All of these are reducible to intellect and desire. For imagination and perception occupy the same ground as intellect; for all of them are discriminative, though they differ according to the distinctions stated elsewhere.
11| Wish, spirit, and appetite are all desire, while choice is common to thought and desire; so that what moves first is the object of desire and the object of thought. But not every object of thought — only the end of things that can be done. Hence it is a thing of this sort among goods that moves, and not every fine thing; for it moves in this way because it is for the sake of something else, and it is the end of things that exist for the sake of something else. One must also posit that the apparent good holds the place of the good, and likewise the pleasant; for the pleasant is an apparent good. So it is clear that what is always moved by what always moves and each of the animals are moved in a similar way, yet also in a different way; and this is why some things are always in motion, while the motion of animals has a limit. What is eternal is fine, and what is truly and primarily good, and not at one time so and at another time not, is more divine and more honorable than to have anything prior to it. The first mover, then, moves without being moved, while desire and the faculty of desire move while being moved. The last of the things moved need not move anything. From this it is also clear why, reasonably enough, locomotion is the last of the kinds of motion that occur among moving things—
12| for the animal is moved and travels by desire or choice, once something has been altered in accordance with perception or imagination. But how is it that, in thinking, an animal at one time acts and at another does not act, and is moved at one time and at another is not moved? Something similar seems to happen also with those who reason and form syllogisms about things that are unchanging. But there the end is a piece of theoretical knowledge (for when one has thought the two premises, one has thought and put together the conclusion), whereas here the conclusion that comes from the two premises is the action — as, for instance, when one thinks that every man ought to walk, and one is oneself a man, one walks at once; or if one thinks that no man ought to walk just now, and one is oneself a man, one at once stays still. And one does both of these, provided nothing prevents or compels otherwise. I ought to make something good, and a house is something good; one makes a house at once. I need covering, and a cloak is a covering; I need a cloak. What I need, I ought to make; I need a cloak; I ought to make a cloak. And the conclusion, "I ought to make a cloak," is an action. And one acts from a starting point: if there is to be a cloak, this must first exist of necessity, and if this, then this; and one does this at once. That the action is the conclusion, then, is clear; but the premises that produce action come about through two kinds — through the good and through the possible. And just as some who ask questions do, so too here thought, even while attending to it, gives no consideration at all to the one premise that is obvious—
13| For example, if walking is good for a man because he is himself a man, he does not dwell on this. That is why, whatever we do without reasoning it out, we do quickly. For whenever a thing acts either by perception toward that for the sake of which, or by imagination, or by intellect, on what it desires, it acts at once; for the actuality of desire takes the place of inquiry or thinking. "I must drink," appetite says; "this here is drink," perception or imagination or intellect has said; at once it drinks. In this way, then, animals are set moving toward motion and action, the last cause of being moved being desire, and this desire coming about either through perception or through imagination and thinking. Of those that desire to act, some act through appetite or spirit, others through desire or wish; some make, others do. And just as automatic puppets are moved when a small motion occurs, the cords being released and striking against one another, and the toy cart, which when ridden moves itself in a straight line and again moves in a circle because it has unequal wheels (for the smaller wheel becomes like a center, as happens with cylinders), so too animals move. For they have organs suited to this: the nature of the sinews and of the bones — the bones playing the part of the wood and iron there, the sinews the part of the cords —
14| when these are released and slackened, the animal moves. Now in the automatic puppets and the toy carts there is no alteration, since if the inner wheels became smaller and then larger again, the same thing would still move in a circle; but in the animal the same part can become both larger and smaller and change its shape, as the parts grow through heat and again contract through cooling and are altered. And it is imaginations, perceptions, and conceptions that cause alteration. For perceptions are, straightaway, themselves certain alterations, while imagination and thinking have the power of the actual things; for in a way the form conceived, of the hot or the cold or the pleasant or the frightening, turns out to be just such as each of the actual things is, which is why people shudder and grow afraid from thinking alone. All these are affections and alterations. And as the body is altered, some parts become larger, others smaller. That a small change occurring at the origin produces great and numerous differences further off is not unclear: for instance, when the rudder shifts by a hair's breadth, a great shift results in the prow. Further, when an alteration occurs around the heart owing to heat or cold or some other such affection, even one of magnitude confined to an imperceptible part, it produces a great difference in the body, in blushing and paling, shivering and trembling, and their opposites.
15| The origin of motion, then, as has been said, is the pursued and the avoided within the sphere of action; and heat and cold follow of necessity upon the thinking and imagining of these. For the painful is to be avoided, the pleasant to be pursued, though in small matters this happening escapes notice, and painful and pleasant things are nearly all accompanied by some cooling or heating. This is clear from the affections. For confidence and fears and sexual excitements and the other bodily pains and pleasures are in some cases, in a particular part, accompanied by heat or cold, in others throughout the whole body; and memories and hopes, since they employ images of these same things, are causes of the same effects, at times less, at times more. So it is reasonable that already the internal parts, and those around the origins of the organic parts, are being worked, changing from solid to fluid and from fluid to solid, and soft and hard into one another. Since these things happen in this way, and further since the passive and the active have the nature we have often stated, whenever it comes about that one thing is active and the other passive, and neither of the two falls short of what its account requires, at once the one acts and the other is acted on.
16| For this reason one thinks, so to speak, at the very moment that one must walk, and walks, unless something else impedes it. For the affections prepare the organic parts fittingly, desire prepares the affections, and imagination prepares desire; and imagination comes about either through thinking or through perception. And it happens together and swiftly because the active and the passive are by nature such as to act on one another. The first mover of the animal must be located in some origin. That the joint is an origin of one part and an end of another has been stated. That is why nature uses it in one sense as one thing, in another sense as two. For when motion occurs from this point, one of the extreme points must be at rest and the other in motion; for it has been said earlier that the mover must brace itself against something at rest. Now the extremity of the upper arm is moved and does not move anything further; and of the bend at the elbow, part is moved along with the whole that is in motion, but there must also be something unmoved, which we say is potentially one point but actually becomes two; so that if the animal were the arm, the origin of the soul that moves it would be there.
17| But since it is possible for something lifeless too to stand in a like relation to the hand, for instance if one were to move a stick held in the hand, it is clear that the soul would not be in either of the extremities, neither in the extremity of the thing moved nor in the other origin. For the piece of wood also has both a beginning and an end in relation to the hand. So on this ground, if the moving origin from the soul is not present in the stick, neither is it in the hand; for the tip of the hand stands in the same relation to the wrist, and this part in turn to the elbow. For there is no difference between parts that have grown on and parts that have not, since the stick becomes, as it were, a detachable part. It follows, then, that the origin must be in no point that is the end of another, nor even if there is something still further outward than that — for instance, of the extremity of the stick the origin is in the hand, and of this, in the wrist; and if it is not even in the hand, because the origin lies still higher, then it is not there either. For further, while the elbow remains still, the whole continuous part below it moves.
18| Since the same holds on the left side as on the right, and opposite parts move at the same time, so that it is not the case that the left moves because the right is at rest, nor again that the right moves because the left is at rest, but the origin always lies above both, the origin of the moving soul must be in the middle; for the middle is the extreme point common to both of the extremities. And this holds similarly for motions from above and below — for instance, motions from the head as against motions from the spine, in animals that have a spine. And this comes about reasonably; for we say the faculty of perception is there too, so that, when the region around the origin is altered through perception and changes, the parts connected to it change along with it, being stretched out and drawn together, so that the motion in animals necessarily comes about through these causes. And the middle part of the body is potentially one, but must actually become several; for the limbs move at the same time from the origin, and while one is at rest the other moves. I mean, for instance, on the line ABC, B is moved, and it moves A. But surely something must remain at rest, if one part is to move and another to be moved.
19| So A, being one in potentiality, will be two in actuality, and hence it must be not a point but some magnitude. But it is possible for C to move at the same time as B, so that both principles within A must, in moving, cause motion. There must, then, be something else besides these that causes the motion, and is itself unmoved. For if the extremities and the principles within A moved against one another, they would simply brace against each other, as if some people, bracing their backs against each other, were to move their legs. But that which moves both must be unmoved, and this is the soul, being distinct from that sort of magnitude, though present within it.
20| 10 According to the account that states the cause of motion, desire is the middle term, which, itself being moved, causes motion; and in ensouled bodies there must be some such body. Now that which is moved but is not naturally constituted to cause motion can be acted on in virtue of another's power; but that which causes motion must have some power and strength. And all animals evidently possess connate breath, and are strong by means of it. What the preservation of the connate breath consists in has been stated elsewhere; and this breath appears to stand to the psychic principle in a way similar to the point at the joints, which both moves and is moved, in relation to the unmoved. Since the principle is in the heart for some animals, and in the analogous part for others, this is why the connate breath also appears to be located there. Whether the breath is always the same or is always coming to be different, let that be a separate account (for the same question arises about the other parts too); but it evidently has a good natural fitness to be a source of motion and to supply strength. And the functions of motion are pushing and pulling, so that the instrument must be capable both of expanding and of contracting.
21| Such is the nature of breath: for it contracts without force, and is forceful and capable of pushing for the same reason, and it has both weight relative to fiery things and lightness relative to their opposites. And what is going to cause motion must be of this sort without undergoing alteration; for natural bodies overpower one another in accordance with their degree of excess, the light being overcome downward by the heavier, the heavy upward by the lighter. We have stated, then, by what moved part the soul causes motion, and for what cause. And one must suppose that the animal is constituted like a well-governed city-state. For in a city-state, once order has been established, there is no need of a separate monarch who must be present at each and every thing that happens; rather each person does his own work as it has been assigned, and this happens after that by custom. And in animals this same thing comes about through nature, and because each part, being so constituted, is naturally suited to do its own work, so that there is no need for soul to be present in each part; rather, the principle being in some one part of the body, the rest live by being attached to it, and do their own work through nature.
22| How, then, animals move with voluntary motions, 11 and through what causes, has been stated; but some of the parts move with involuntary motions too, and most with motions that are not voluntary. I mean by involuntary, for example, the motion of the heart and of the genitals (for these often move at the appearance of something, yet not at the command of intellect); and by not voluntary, for example, sleep and waking and breathing, and any other such motions. For none of these is under the sole control of either imagination or desire; rather, since animals are necessarily altered by natural alteration, and as the parts are altered some grow and some diminish, so that motions and changes already occur, these changes being naturally disposed to follow one another. The causes of these motions are states of heat and cold, both those coming from outside and those that are internal and natural. And even the motions of the aforesaid parts that occur contrary to reason come about when an alteration has coincided with them. (For thought and imagination, as has been said before, bring on the things that are productive of these affections; for they bring on the forms of the things that are productive.) These effects are most conspicuously produced in these parts because each of the parts is, as it were, a separate animal in its own right.
23| Now the cause with respect to the heart is clear, for it holds the principles of the senses; and as for the generative part, that it is of this same sort is shown by a sign: for the power of the seed issues from it as though it were itself a kind of animal. And the motions reasonably occur both from the parts to the principle and from the principle to the parts, and in this way they arrive at one another. For one must think of A as the principle. The motions along each of the elements marked in the diagram arrive at the principle, and from the principle, as it moves and changes—since it is many things in potentiality—the principle of B goes to B, that of C to C, and that of both goes to both. And the motion goes from B to C by way of going from B to A, as to a principle, and from A to C, as from a principle. And the reason why sometimes, when the same things are thought, the motion contrary to reason occurs in the parts, and sometimes not, is that sometimes the passive matter is present in a given quantity or quality, and sometimes it is not present in that quantity or quality. Concerning the parts of each of the animals, then, and concerning the soul, and further concerning perception and sleep and memory and motion common to all, we have stated the causes.
24| It remains to speak about generation.