Aristotle · a new plain-English translation from the original language
1| On the Parts of Animals, Book 1. [section 1] Concerning every study and method, whether more humble or more honorable, there appear to be two kinds of competence: one it is proper to call knowledge of the subject, the other something like an education in it. For it belongs to a person educated in this way to be able to judge shrewdly whether what a speaker sets forth is well done or not well done. For we think that the person educated in general is of just this sort, and that being educated is being able to do what has been said. Except that we consider such a person a critic of virtually everything, being one in number, while the other is a critic concerning some definite nature — for there might be some other person disposed in the same way as the one described, but with respect to a part. [section 2] So it is clear that for the inquiry concerning nature too there must be certain standards of this kind, in reference to which one will accept the manner of the things being demonstrated, apart from how the truth stands, whether it is this way or that. I mean, for example, whether one ought to take up each single substance and mark it off concerning itself — treating in turn, say, the nature of man or of the lion or of the ox or of some other thing, each by itself — or whether one ought to posit first the attributes that belong to all of them in common, according to some common feature. For many of the same attributes belong to many genera that are different from one another — for instance sleep, respiration, growth, decay, death, and
2| and, in addition to these, all such of the remaining affections and states as are of this kind; for it is unclear and undefined to speak of these now. [section 3] It is evident too that if we speak part by part concerning many animals we shall often say the same things repeatedly; for each of the things mentioned belongs to horses and dogs and men alike, so that if one speaks of each of the attributes individually, one will often be forced to speak about the same things — as many as belong alike to animals differing in species, while themselves admitting no differentia. But there are perhaps other things for which it holds that they fall under the same category, yet differ by a differentia of species — for example the locomotion of animals; for this does not appear to be one in species, since flying, swimming, walking, and crawling differ from one another. [section 4] For this reason one must not fail to notice how the inquiry is to be conducted — I mean, whether one should treat first what is common by genus, and only afterward consider the properties peculiar to each, or take up each thing individually straightaway. For at present this has not been determined, nor has what is now about to be said. For example, whether, just as the mathematicians demonstrate the facts about astronomy, so too the student of nature ought first to study the observed facts concerning animals and the parts belonging to each, and only then go on to state the reason why and the causes, or proceed in some other way. Furthermore, since we see that there are several causes at work in natural generation — for instance that for the sake of which, and that from which the principle of motion arises — we must determine, concerning these too, which is naturally first and which second.
3| [section 5] The first appears to be the one we call for the sake of something; for this is an account, and the account is a principle alike in things constituted by craft and in things constituted by nature. For whether by thought or by perception, the doctor, having marked out health, and the builder, having marked out the house, render the accounts and the causes of each thing they make, and why it must be made in this way. And the that-for-the-sake-of-which, and the fine, are present more in the works of nature than in those of craft. But necessity does not belong in the same way to all things that are according to nature — the thing to which almost everyone tries to reduce their accounts, without having distinguished in how many senses the necessary is spoken of. [section 6] One kind belongs simply to eternal things, the other, the necessity that follows from a hypothesis, belongs also to all things that come to be, just as in things made by craft — for instance a house and anything else of this sort. For if there is to be a house, or any other end, it is necessary that the matter be of a certain kind; and this must come to be and be moved first, then this, and in this way in succession up to the end and that for the sake of which each thing comes to be and is. And it is the same way too in things that come to be by nature. But the manner of demonstration and of necessity is different in natural science from that in the theoretical sciences. This has been discussed elsewhere.
4| [section 7] For the principle is, in the one case, the thing that is, in the other, the thing that will be. For since health, or man, is of such-and-such a kind, it is necessary that this exist or come to be — but it is not the case that, because this exists or has come to be, that other thing exists or will exist of necessity. Nor is it possible to fasten the necessity of such a demonstration onto something eternal, so as to say, because this is, that this is. These matters too have been determined elsewhere — both to what things this necessity belongs, and which cases convert, and for what cause. [section 8] One must also not fail to notice whether it is fitting to speak, as those before us conducted their inquiry, of how each thing is naturally constituted to come to be, rather than of how it is. For this differs from that by no small amount. It seems that one must begin from this point, just as we said before, that first the observed facts about each kind must be grasped, and only then must the causes of these be stated, along with the facts about their coming-to-be; for this holds even more in the case of building — for this holds in the case of house-building too: it is rather that these things come about because the form of the house is of such-and-such a kind, than that the house is of such-and-such a kind because it comes to be in this way. [section 9]
5| For coming-to-be is for the sake of substance, and not substance for the sake of coming-to-be. This is why Empedocles was not right when he said that many things belong to animals because it happened that way in their coming-to-be — for instance, that the spine is as it is because it happened to be broken when the embryo was twisted. He failed to notice, first, that the seed once formed must already possess the appropriate capacity, and second, that the thing which produced it existed earlier, not only in account but also in time; for man begets man, so that it is because that parent is of such-and-such a kind that this offspring's coming-to-be turns out as it does. [section 10] The same holds too for things that seem to come about spontaneously, just as for things made by craft; for some things come about spontaneously that are the same as things that come about by craft — health, for example. Now in the case of some of these, the productive cause pre-exists and is similar in kind, as with statue-making; for a statue does not come about spontaneously. But craft is the account of the product, existing apart from the matter. And things that come about by chance come about in the same way as things that come about by craft; for they come about just as craft would have them. Hence it is best of all to say that, since this was what it is to be a man, therefore these parts belong to him — for it is not possible for him to exist without these parts. But if not that, then at least that it is nearest to this, and either that it is altogether impossible otherwise, or that it is well for it to be so. And these things follow. And since a thing is of such a kind, it is necessary that its coming-to-be occur in this way and be of this sort. This is why, among the parts, this one comes to be first, then this one. And this holds in the same way, alike, for all things constituted by nature.
6| [section 11] Now the ancients, the first to philosophize about nature, inquired into the material principle and cause of this kind — what it is and of what sort, and how the whole comes to be out of it, and by what moving cause, such as strife or love or intellect or spontaneity, while the underlying matter has of necessity a certain nature — fire, for instance, being hot, and earth cold, and the one light, the other heavy. For it is in this way that they generate the world-order as well. [section 12] And they speak likewise about the coming-to-be of animals and plants — saying, for example, that as water flowed in the body, a stomach cavity came to be, along with every receptacle for nourishment and residue, and that as breath passed through, the nostrils were broken open. Air and water are the matter of bodies; for it is out of bodies of this sort that everyone constitutes nature. But if man and the animals and their parts exist by nature, then one would need to speak about flesh and bone and blood and all the uniform parts. [section 13]
7| Similarly too with the non-uniform parts, such as the face, the hand, the foot: we must state both what each of them is and by what capacity. For it is not enough to say what they are made of, for instance fire or earth, just as, if we were speaking about a bed or something else of that kind, we would try rather to define its form than its matter -- the bronze or the wood -- or if not that, at least the form of the composite; for a bed is this form in this matter, a thing of such-and-such a kind, so that we would have to speak also about its shape, and what sort its figure is. For the nature that accords with the shape is more authoritative than the material nature. [section 14] If, then, each of the animals and their parts were what it is by shape and color, Democritus would be right; for he appears to have supposed just this. At any rate he says it is plain to everyone what sort of shape a human being has, since a human being is recognizable by shape and color. And yet a corpse too has the very same shape and form, but all the same it is not a human being. [section 15] Further, it is impossible for a hand to be in just any condition whatever -- say, made of bronze or of wood -- except homonymously, like a painted doctor. For it will not be able to perform its own function, just as stone flutes cannot perform their function, nor can a painted doctor perform his. In the same way, none of the parts of a dead body are any longer parts of that kind either -- I mean, for example, an eye or a hand. So the statement was made far too simply, in the same way as if a carpenter were speaking about a wooden hand.
8| [section 16] For this is just how the natural philosophers, too, speak of the comings-to-be and the causes of shape -- by what capacities the things were fashioned. But perhaps the carpenter will name the axe or the drill, while the natural philosopher names air and earth; yet the carpenter's answer is better. For it will not be enough for him to say only this much, that when the tool struck, one part became hollow and another flat; rather he must say why he made the blow of that kind, and for the sake of what -- he will state the cause, namely so that the shape might come to be such-and-such. It is plain, then, that they do not speak correctly, [section 17] and that we must speak of the animal as being of such-and-such a kind, and speak about that, both what it is and of what sort it is, and likewise about each of its parts -- just as we must about the form of the bed. If, then, this is soul, or a part of soul, or not without soul (at any rate, once the soul has departed, it is no longer an animal, and none of the parts remains the same, except in shape only, just as the things told of in myth are turned to stone) -- if, then, this is so, it would belong to the natural philosopher to speak of and know about the soul, and if not about all soul, at least about that very part of it in virtue of which the animal is of such a kind; and to know what the soul is, or this part of it, and about the attributes that belong in accordance with such a substance of it -- especially since nature is spoken of and exists in two senses, the one as matter, the other as substance. And this is so both as the mover and as
9| the end. And that which is of this kind for the animal is either the whole soul or some part of it. So that on this account too the student of nature would have to speak about the soul rather than about the matter, inasmuch as the matter is nature because of the soul rather than the reverse. For indeed wood is a bed and a tripod because it is these things potentially. [section 18] One might raise a difficulty on looking at what has just been said, whether it belongs to natural science to speak about the whole soul or only about some part of it. For if about the whole, then no philosophy at all is left outside natural science. For intellect is of the objects of intellect. So that the knowledge of nature would be about all things; for it belongs to the same science to study intellect and the object of intellect, if indeed they are correlative, and the same inquiry covers all things that are correlative, just as also with perception and the objects of perception. Or is it rather that not every soul is a principle of motion, nor are all its parts, but the principle of growth is that part which belongs also to plants, of alteration the perceptive part, and of locomotion something else again, and not the intellective part? For locomotion belongs also to other animals, but thought to none of them.
10| [section 19] It is plain, then, that we should not speak about the whole soul; for not even the whole soul is nature, but some one part of it, or more than one. Further, natural science cannot be a study of anything arrived at by abstraction, since nature does everything for the sake of something. For it appears that, just as art is present in the things made by art, so in the things themselves there is some other principle and cause of that kind, which we have in the way we have hot and cold from the universe. That is why it is more reasonable that the heavens, if they came to be, came to be by such a cause, and exist because of such a cause, rather than the mortal animals do; for what is ordered and determinate appears far more in the heavenly things than around us, while what is now one way and now another, and as it happens, appears more among mortal things. [section 20] But some say that each of the animals exists and came to be by nature, while the heavens were constituted as they are by chance and spontaneity -- and yet in the heavens nothing whatever appears to result from chance and disorder. But everywhere we say that this is for the sake of that, wherever some end appears toward which the motion proceeds when nothing hinders it. So it is plain that there is something of this kind, which we in fact call nature. For it is certainly not the case that just anything comes to be from a given seed, but this from this; nor is just any seed from just any body.
11| [section 21] The seed, then, is a principle and productive of what comes from it. For these things exist by nature: at any rate, the thing grows from this. But further, prior even to this is that from which the seed itself comes; for the seed is a coming-to-be, but the end is a substance. And prior to both is that from which the seed is derived. For seed is spoken of in two ways, as "from which" and "of which": for it is the seed of that from which it departed -- for instance, of a horse -- and also of that which will come from it -- for instance, of a mule -- though not in the same way, but in the way stated for each. [section 22] Further, the seed is something potential; and we know how a potentiality stands in relation to actuality. There are, then, these two causes, the that-for-the-sake-of-which and the from-necessity; for many things come to be because it is necessary. Perhaps one might raise a difficulty as to what kind of necessity those mean who speak of things coming to be from necessity: for neither of the two ways distinguished in works on philosophy can apply here. But there is, at least among things that undergo coming-to-be, a third way; for we say that food is necessary in neither of these two ways, but because it is impossible to exist without it. This is necessity as it were from a hypothesis: just as, since one must split wood with an axe, the axe must be hard, and if hard, then of bronze or iron, so too, since the body is an instrument (for each of the parts exists for the sake of something, and similarly
12| ...and also the whole), it is necessary that it be of such a kind and made of such materials, if that end is to come about. [section 23] That there are, then, two modes of causation, and that those who speak of them must try to give both if possible, or failing that, must at least try to make this clear, and that all who fail to state this say virtually nothing about nature—for nature as a principle is more truly form than matter. Empedocles too, in places, stumbles onto this, drawn on by the truth itself, and is forced to say that substance and nature are the account, as when he tells what bone is: for he does not call it any one of the elements, nor two nor three, nor all of them, but the ratio of their mixture. [section 24] It is clear, then, that flesh too is of the same character, and likewise each of the other such parts. The reason the earlier thinkers did not arrive at this approach is that essence and the defining of substance did not yet exist; Democritus was the first to touch on it, though not because it was necessary for the study of nature, but because he was carried along by the subject itself. Under Socrates this was advanced further, but the inquiry into nature ceased, and those who pursued philosophy turned instead toward useful virtue and political matters.
13| [section 25] The demonstration must be given in this way: for instance, that respiration is for the sake of such-and-such, and that this comes about of necessity through such-and-such causes. Necessity sometimes means that, if that which is for the sake of something is to be, these things are necessary for it to have; and sometimes it means that things simply are so and are naturally constituted so. For it is necessary that the hot go out and then come in again by striking back, and that the air flow in. This last point is already a matter of necessity. As the internal heat strikes back in the process of cooling, the entry and exit of the outside air follow. This, then, is the manner of the method, and these and things like them are the matters concerning which one must grasp the causes. [section 1] 2 Some thinkers try to grasp the particular by dividing the genus into two differentiae. This is in one respect not easy, and in another impossible. For of some genera there will be only one differentia, and the rest superfluous—for example footed, two-footed, cleft-footed, footless: for this alone is the governing one. If not, the same thing must often be said twice over. Further, it is proper not to tear apart any single genus, as for instance putting some birds in this division and others in another, the way the published divisions do; for there it happens that some are divided off together with the water animals,
14| [section 2] 2 and others in a different genus. Now by this one resemblance the name 'bird' is given, by another 'fish.' But there are other resemblance-classes that are nameless—for instance the blooded and the bloodless—for on neither of these has a single name been imposed. If, then, none of the things of the same kind ought to be torn apart, division into two would be futile: for those who divide this way are forced to separate and tear apart—for instance, of the many-footed creatures, some fall among the land animals and others among the water animals. [section 1] 3 Further, it is necessary to divide by privation, and those who divide by dichotomy do so divide. But there is no differentia of privation as privation: for it is impossible for there to be species of what is not, such as of footlessness or of winglessness, as there are of having wings and having feet. But there must be species of the universal differentia; for if there will not be, why would it belong to the universal class and not to the particular? Now of differentiae, some are universal and have species, such as featheredness: for one kind of feather is unsplit and another is split. And footedness likewise: one kind is many-cleft, another two-cleft, as in the cloven-hoofed, and another uncleft and undivided, as in the single-hoofed.
15| [section 2] 2 It is difficult, then, to distinguish differentiae of this kind, of which there are species, in such a way that any given animal falls under them and not under several at once—for instance winged and wingless (for both belong to the same animal, as in the ant, the glow-worm, and certain others)—but hardest of all, or impossible, in the case of the bloodless animals. For it is necessary that each of the particular differentiae belong to some one of them, and so also its opposite. [section 3] 3 But if it is not possible for a single, indivisible species of substance to belong to things that differ in species, but it will always have a differentia—as bird differs from man (for two-footedness is different and distinct in each)—then even if both are blooded, the blood is different; or else blood must be posited as no part of the substance. But if it is as I said, one differentia will belong to two distinct animals. And if this is so, it is clear that privation cannot be a differentia. The differentiae will be equal in number to the indivisible animals, if these are indivisible and the differentiae are indivisible, and none is common to more than one. But if it is possible for a differentia to belong without being common, yet still be indivisible, it is clear that, at least in respect of the common one, animals that differ in species fall under the same thing. So it is necessary, if the differentiae are proper to the classes under which all the indivisible animals fall, that none of them be common. Otherwise, being different animals, they will fall under the same one.
16| [section 4] 4 It is necessary that the same indivisible thing not fall under one differentia and another among those divided off, nor that different things fall under the same one, while all fall under the same set. It is clear, then, that it is not possible to grasp the indivisible species in the way that those who divide the animals, or any other genus, into two do. For according to their method it is necessary that the ultimate differentiae be equal in number to all the animals indivisible in species. For if there is some genus whose first differentiae are the white things, and of each of these there are others, and so on forward to the indivisible ones, the last differentiae will amount to some multiple obtained by repeated doubling from one; and the species will be just as many. [section 5] 5 The differentia is the form in the matter. For no part of an animal exists without matter, nor does matter alone make one; for not everything that has a body will be an animal, nor will any of its parts, as has often been said. Further, one must divide by what is in the substance, and not by what is accidental in its own right—for instance, if someone were to divide figures on the ground that some have angles equal to two right angles, and others to more;
17| [section 6] 6 for having angles equal to two right angles is an accident of the triangle. Further, one must divide by opposites. For opposites differ from one another, such as whiteness and blackness, and straightness and curvedness. If, then, one set of things is distinct by one differentia, one must divide by the opposite, and not divide the one by inclination and the other by color. Besides these, ensouled things must be divided by the functions common to body and soul, as, for instance, among those mentioned, walking and flying: for there are certain genera to which both belong, which are both winged and wingless, as the genus of ants. Nor should one divide by the wild and the tame: for it would seem that in doing so one is dividing the same species twice over, since practically all the tame animals happen also to be wild, such as men, horses, cattle, dogs in India, pigs, goats, sheep. Of each of these, if the name is used equivocally, it has not been divided apart at all; but if these are one in species, then wild and tame cannot be a differentia. [section 7] 7 In general, it is necessary that this happen to anyone who divides by a single differentia at a time. Instead, one must try to grasp the animals by genera, as the many who have marked off the genus of bird and of fish have indicated. Each of these has been settled by many differentiae, not according to dichotomy. For in that way either it is not possible to grasp them at all (since the same thing falls under several divisions, and opposites fall under the same one), or there will be only a single differentia, and this either simple, or, in the last case, formed by combination
18| ...will be the species. [section 8] But if one does not take a differentia of a differentia, then, just as one makes the account one by a connective, so too one must make the division continuous. I mean, for example, what happens to those who divide: featherless on one side, feathered on the other; and of the feathered, tame on one side, wild on the other, or white on one side, black on the other -- for tame is not a differentia of feathered, nor is white, but each is the starting point of a different differentia; there it is accidental. That is why one must immediately divide the one thing into many, as we say. For in this way the privations will produce a differentia, but in the dichotomy they will not. [section 9] That it is not possible, when dividing the genus in two, to arrive at any of the species taken individually, as some have supposed, is clear also from the following. For it is impossible for there to be a single differentia of things divided individually, whether one takes them as simple or as compounded. By simple I mean whether it has a differentia or not, for example many-toed as opposed to cloven-footed; by compounded I mean, for example, cloven-footedness. For this is what the continuity of the differentiae taken from the genus in the course of the division aims at, that the whole be some one thing. But it happens, because of the way we speak, that only the last differentia seems to be the differentia -- for example many-toed or two-footed -- while footed and many-footed seem superfluous.
19| [section 10] That there cannot be several such differentiae is clear: for one who proceeds always arrives at the last differentia, but not at the final one and the species. This last differentia is either cloven-footed alone, or the whole complex, if one is dividing to reach man -- for example if one puts together footed, two-footed, cloven-footed. If man were cloven-footed alone, this would then be a single differentia. But since he is not, there must be many differentiae, not falling under a single division. But indeed it is not possible for several such to fall under a single dichotomy of the same thing -- rather one must end at one point along one line of division. So it is impossible to grasp any of the individual animals by dividing in two. [section 1] One might raise the difficulty why people did not, from the start, gather both kinds under one name and call them a single genus, embracing both the water animals and the winged animals -- for there are some affections common to these and to all the other animals as well. But all the same, the division has been made correctly in this way. For whatever genera differ from one another by excess and by the more and the less are yoked together under one genus, while whatever have the analogous feature are kept apart. I mean, for example, one bird differs from another bird by the more, or by excess (one has long feathers, the other short), but fish differ from birds by analogy (what feather is to the one, scale is to the other). But it is not easy to do this in every case, for most animals undergo the same thing only by analogy.
20| [section 2] Since substances are the last species, and these are undifferentiated in species -- for example Socrates, Coriscus -- one must state what belongs to them universally before saying the same thing many times over, as has been said. Universals are common, for we call universal what belongs to several things. There is a difficulty as to which of the two one ought to treat. For insofar as substance is what is indivisible in species, it would be best, if one were able, to study the individuals and what is indivisible in species separately -- as one does with man, so likewise not with bird in general (for this genus has species), but with whatever individual bird, for example sparrow or crane or something of that kind. But insofar as it will turn out that one says the same thing about the same affection many times over, because it belongs commonly to several things, in this respect it is quite absurd and tedious to speak of each separately. [section 3] Perhaps, then, it is correct
21| to speak in common by genera of whatever is well said once people have marked out the boundaries, and which has one common nature, with species among them not far apart -- bird and fish, and anything else that, though nameless, likewise embraces in its genus the species within it; but of whatever is not of this kind, to speak of each individually, as with man and anything else of that sort. And the genera are marked out, roughly speaking, by the shapes of the parts and of the whole body, when these have a similarity -- for example, the genus of birds is affected in the same way in relation to itself, and so is that of fish, and the soft-bodied animals, and the shelled animals. For their parts differ from one another not by an analogous similarity -- as in man and fish, where bone stands to spine -- but rather by bodily affections, such as largeness and smallness, softness and hardness, smoothness and roughness, and things of that kind, and in general by the more and the less. [section 4] How, then, one ought to receive the method concerning nature, and in what way the study of these things would proceed most easily and by the right path, and further, concerning division, in what way it is possible for those pursuing it to grasp it usefully, and why dichotomizing is in one respect impossible and in another empty, has been stated. Now that these points have been settled, let us speak of what comes next, making the following our starting point.
22| [section 1] Of the substances constituted by nature, some are ungenerated and imperishable for all eternity, while others share in generation and destruction. Now it has fallen out that, concerning the former -- which are honorable and divine -- we have less scope for study (for both the starting points from which one might investigate them, and the things about which we long to know, the things evident to perception are altogether few); but concerning the perishable plants and animals we are better off with respect to knowledge, because of our familiarity with them, for one could grasp many facts about each of the kinds that exist, [section 2] if one is willing to work hard enough at it. Each has its own charm. For as to the former, even if we grasp only a little of them, still, because of the honor of the knowledge, it is more pleasant than knowing everything around us, just as catching sight of even a chance and small part of those we love is more pleasant than a precise view of many other great things; while as to the latter, because we come to know more of them and more fully, they take the superiority in point of knowledge, and further, because they are nearer to us and more akin to our nature, they offer something in exchange, weighed against the philosophy concerning divine things. [section 3]
23| Since we have gone through those subjects, stating what appears to us to be the case, it remains to speak of the nature of animals, leaving out nothing, so far as we are able, whether less honorable or more honorable. For even in the study of those animals which are not pleasing to perception, the nature that fashioned them nonetheless provides extraordinary pleasures to those able to know the causes and who are philosophers by nature. Indeed it would be unreasonable and absurd if, when looking at images of them, we take delight because we are at the same time contemplating the craft that made them -- for example painting or sculpture -- while not caring more for the study of the things constituted by nature themselves, when we are in fact able to observe their causes. [section 4] For this reason one must not be childishly squeamish about examining the less honorable animals. For in all natural things there is something marvelous. And just as Heraclitus is said to have spoken to the strangers who wished to meet him, who, when they approached and saw him warming himself at the oven, stopped short (for he urged them to come in without fear, since there were gods there too), so too one must approach the inquiry about each animal without disgust, since in all of them there is something natural and beautiful. For that which is not by chance but for the sake of something belongs to the works of nature, and above all, that for the sake of which a thing is put together or has come to be, as its end, occupies the place of the beautiful.
24| [section 5] If someone has come to think that the study of the other animals is beneath his dignity, he ought to think the same about the study of himself as well; for it is not possible to look at what the human race is composed of — blood, flesh, bones, veins, and parts of that kind — without a great deal of distaste. Likewise, one must recognize that a person discussing any part or any instrument whatsoever should not be making his account concerned with the matter, nor for the sake of the matter itself, but for the sake of the whole shape — just as with a house, the account is about the house, not about bricks and mud and timber. And so too the student of nature should be concerned with the composition and the whole substance, not with those things which never occur separated from their own substance. [section 6] It is necessary first to distinguish the attributes that belong to each genus, as many as belong to all the animals in their own right, and after that to try to distinguish their causes. Now it has been said before that many features are common to many animals — some without qualification, such as feet, feathers, scales, and affections of the same kind as these, and others by analogy. By 'analogy' I mean that some animals have a lung, while others have no lung but instead have, in place of it, something else that plays for them the role the lung plays for those that have one; and some have blood, while others have the analogous thing possessing the same power that blood has for the blooded animals.
25| [section 7] As for speaking about each of the particular animals separately — we said earlier too that this will often result in saying the same things again and again, since we would be speaking about all the features that belong to them, and the same features belong to many of them. Let this point, then, be settled in this way. Now since every instrument exists for the sake of something, and each of the parts of the body exists for the sake of something, and that for the sake of which is some action, it is clear that the body as a whole too has been put together for the sake of some complete action. For sawing does not exist for the sake of the saw, but the saw for the sake of sawing; for sawing is a kind of use. So the body too is in some way for the sake of the soul, and the parts are for the sake of the functions toward which each is naturally suited. [section 8]
26| We must therefore first speak of the actions — those common to all, those according to genus, and those according to species. By 'common' I mean those that belong to all animals; by 'according to genus,' those where we see the differences between them holding by way of degree relative to one another — for instance, I speak of bird according to genus, but of man according to species, and in general anything that, taken according to the universal account, exhibits no differentia. For some things have what is common by analogy, others according to genus, others according to species. Now whatever actions exist for the sake of other actions, it is clear that the parts whose actions these are stand apart from one another in the same way the actions do. Likewise, if some actions happen to be prior to and the end of other actions, each of the parts whose actions are of that sort will stand in the same relation; and a third case, those parts which must exist given that the others exist. [section 9] By affections and actions I mean coming-to-be, growth, mating, waking, sleep, locomotion, and whatever other such things belong to animals; by parts I mean nose, eye, and the face as a whole, each of which is called a limb. And likewise with the rest. Let this much be said, then, about the manner of our method; let us now try to state the causes, both of the common features and of the particular ones, beginning, as we determined, first from the first things.