by Aristotle
The fittest mode, then, of treatment is to say, a man has such and such parts, because the essence of man is such and such, and because they are necessary [35] conditions of his existence, or, if we cannot quite say this then the next thing to it, namely, that it is either quite impossible for a man to exist without them, or, at any [640b1] rate, that it is good that they should be there. And this follows: because man is such and such the process of his development is necessarily such as it is; and therefore this part is formed first, that next; and after a like fashion should we explain the generation of all other works of nature.
[5] Now that with which the ancient writers, who first philosophized about nature, busied themselves, was the material principle and the material cause. They inquired what this is, and what its character; how the universe is generated out of it, and by what motor influence, whether, for instance, by strife or love or mind or spontaneous action, the substratum of matter being assumed to have of necessity a certain nature—fire, for instance, to have a hot nature, earth a cold one; the former [10] to be light, the latter heavy. For even the genesis of the universe is thus explained by them. After a like fashion do they deal also with the development of plants and of animals. They say, for instance, that the water contained in the body causes by its currents the formation of the stomach and the other receptacles of food or of [15] excretion; and that the breath by its passage breaks open the outlets of the nostrils; air and water being the materials of which bodies are made; for all represent nature as composed of such or similar substances.
But if men and animals and their several parts are natural phenomena, then the natural philosopher must take into consideration flesh, bone, blood, and all the other homogeneous parts; not only these, but also the heterogeneous parts, such as [20] face, hand, foot; and must examine how each of these comes to be what it is, and in virtue of what force. For it is not enough to say what are the stuffs out of which an animal is formed, to state, for instance, that it is made of fire or earth—if we were discussing a couch or the like, we should try to determine its form rather than its matter (e.g. bronze or wood), or if not, we should give the matter of the whole. For a [25] couch is such and such a form embodied in this or that matter, or such and such a matter with this or that form; so that its shape and structure must be included in our description. For the formal nature is of greater importance than the material nature.
Does, then, configuration and colour constitute the essence of the various [30] animals and of their several parts? For if so, what Democritus says will be correct. For such appears to have been his notion. At any rate he says that it is evident to every one what form it is that makes the man, seeing that he is recognizable by his shape and colour. And yet a dead body has exactly the same configuration as a [35] living one; but for all that is not a man. So also no hand of bronze or wood or constituted in any but the appropriate way can possibly be a hand in more than [641a1] name. For like a physician in a painting, or like a flute in a sculpture, it will be unable to perform its function. Precisely in the same way no part of a dead body, such I mean as its eye or its hand, is really an eye or a hand. What he says, then, is [5] too simple—it is much the same as if a woodcarver were to insist that the hand he had cut out was really a hand. Yet the physiologists, when they give an account of the development and causes of the animal form, speak very much like such a craftsman. What are the forces by which the hand or the body was fashioned into its shape? The woodcarver will perhaps say, by the axe or the auger; the physiologist, by air and by earth. Of these two answers the woodcarver’s is the better. For it is not [10] enough for him to say that by the stroke of his tool this part was formed into a concavity, that into a flat surface; but he must state the reasons why he struck his blow in such a way as to effect this, and for the sake of what he did so; namely, that the piece of wood should develop eventually into this or that shape. It is plain, then, that they are wrong, and that the true method is to state what the characters are [15] that distinguish the animal—to explain what it is and what are its qualities—and to deal after the same fashion with its several parts; in fact, to proceed in exactly the same way as we should do, were we dealing with the form of a couch.
If now the form of the living being is the soul, or part of the soul, or something that without the soul cannot exist; as would seem to be the case, seeing at any rate that when the soul departs, what is left is no longer an animal, and that none of the parts remain what they were before, excepting in mere configuration, like the [20] animals that in the fable are turned into stone; if, I say, this is so, then it will come within the province of the natural scientist to inform himself concerning the soul, and to treat of it, either in its entirety, or, at any rate, of that part of it which constitutes the essential character of an animal; and it will be his duty to say what a soul or this part of a soul is; and to discuss the attributes that attach to this essential character, especially as nature is spoken of—and is—twofold, as matter and as [25] substance; nature as substance including both the motor cause and the final cause. Now it is in the latter of these two senses that either the whole soul or some part of it constitutes the nature of an animal; and inasmuch as it is the presence of the soul that enables matter to constitute the animal nature, much more than it is the [30] presence of matter which so enables the soul, the inquirer into nature is bound to treat of the soul rather than of the matter. For though the wood of which they are made constitutes the couch and the tripod, it only does so because it is potentially such and such a form.
What has been said suggests the question, whether it is the whole soul or only some part of it, the consideration of which comes within the province of natural [35] science. Now if it be of the whole soul that this should treat, then there is no place for any other philosophy beside it. For as it belongs in all cases to one and the same science to deal with correlated subjects—one and the same science, for instance, deals with sensation and with the objects of sense—and as therefore the intelligent [645b1] soul and the objects of intellect, being correlated, must belong to one and the same science, it follows that natural science will have to include everything in its province. [5] But perhaps it is not the whole soul, nor all its parts collectively, that constitutes the source of motion; but there may be one part, identical with that in plants, which is the source of growth, another, namely the sensory part, which is the source of change of quality, while still another, and this not the intellectual part, is the source of locomotion. For other animals than man have the power of locomotion, but in none but him is there intellect. Thus then it is plain that it is not of the whole soul that we have to treat. For it is not the whole soul that constitutes the animal nature, [10] but only some part or parts of it. Moreover, it is impossible that any abstraction can form a subject of natural science, seeing that everything that nature makes is for the sake of something. For just as art is present in the products of art, so in the things [15] themselves there is evidently an analogous cause or principle derived like the hot and the cold from the environing universe. And that the heaven, if it had an origin, was generated and is maintained by such a cause, there is therefore even more reason to believe, than that mortal animals so originated. For order and definiteness are much more plainly manifest in the celestial bodies than in our own frame; while [20] change and chance are rather characteristic of the perishable things of earth. Yet there are some who, while they allow that every animal exists and was generated by nature, nevertheless hold that the heaven was constructed to be what it is by chance and spontaneity; the heaven, in which not the faintest sign of chance or of disorder is discernible. Again, whenever there is plainly some final end, to which a motion [25] tends should nothing stand in the way, we always say that the one is for the sake of the other; and from this it is evident that there must be something of the kind, corresponding to what we call nature. For a given seed does not give rise to any chance living being, nor spring from any chance one; but each springs from a definite parent. And thu
s it is that from which the seed comes which is the origin and fabricator of its offspring. For these it is by nature, the offspring being at any [30] rate that which in nature will spring from it. At the same time the offspring is prior to the seed; for the seed is a coming into being, the end a substance. Prior, however, to both is the organism from which the seed was derived. For we speak of seeds in two ways, mentioning that from which it comes and that to which it gives rise: it is both the seed of that from which it came, of the horse, for instance, and the seed of the organism that will eventually arise from it, of the mule, for example—the seed [35] of both, though in different ways as here set forth. Moreover, the seed is potentially something, and the relation of potentiality to actuality we know.
There are then two causes, namely, necessity and the final end. For many [642a1] things are produced, simply as the results of necessity. It may, however, be asked, of what mode of necessity are we speaking when we say this. For it can be of neither of [5] those two modes which are set forth in the philosophical treatises. There is, however, the third mode, in such things at any rate as are generated. For instance, we say that food is necessary in neither of the two modes, but because an animal cannot possibly do without it. This third mode is what may be called hypothetical necessity. For if a piece of wood is to be split with an axe, the axe must of necessity [10] be hard; and, if hard, must of necessity be made of bronze or iron. Now exactly in the same way the body, since it is an instrument—for both the body as a whole and its several parts individually are for the sake of something—if it is to do its work, must of necessity be of such and such a character, and made of such and such materials.
It is plain then that there are two modes of causation, and that both of these must, so far as possible, be taken into account, or that at any rate an attempt must [15] be made to include them both; and that those who fail in this tell us in reality nothing about nature. For nature of an animal is a first principle rather than matter. There are indeed passages in which even Empedocles hits upon this, and following the guidance of fact, finds himself constrained to speak of the ratio as constituting [20] the substance and nature of things. Such, for instance, is the case when he explains what is a bone. For he does not say it is this one element, or those two or three elements, or a compound of all the elements, but states the ratio of their combination. As with a bone, so manifestly is it with the flesh and all other similar parts.
The reason why our predecessors failed to hit on this method of treatment was, [25] that they were not in possession of the notion of essence, nor of any definition of substance. The first who came near it was Democritus, and he was far from adopting it as a necessary method in natural science, but was merely brought to it by constraint of facts. In the time of Socrates a nearer approach was made to the method. But at this period men gave up inquiring into nature, and philosophers diverted their attention to political science and to the virtues which benefit [30] mankind.
Of the method itself the following is an example. In dealing with respiration we must show that it takes place for such or such a final object; and we must also show that this and that part of the process is necessitated by this and that other stage of it. By necessity we shall sometimes mean that the requisite antecedents must be there, if the final end is to be reached; and sometimes that things are thus and so by nature. For the alternate discharge and re-entrance of heat and the inflow of air are necessary—that is necessary; and as the internal heat resists in the process of [642b1] cooling, the entrance and exit of the external air occur.
In the foregoing we have an example of the method which we must adopt, and also an example of the kind of phenomena, the causes of which we have to investigate.
[5] 2 · Some writers propose to reach the ultimate forms of animal life by dividing the genus into two differences. But this method is often difficult, and often impracticable.
Sometimes one differentia is sufficient by itself, and the others are mere surplusage. Thus in the series Footed, Two-footed, Cleft-footed, the last term alone is significant, and to append the others is only an idle iteration.
[10] Again it is not permissible to break up a natural group, Birds for instance, by putting its members under different bifurcations, as is done in the published dichotomies, where some birds are ranked with animals of the water, and others placed in a different class. The group Birds and the group Fishes happen to be named, while other natural groups have no names; for instance, the groups that we [15] may call Sanguineous and Bloodless are not known popularly by any one name. If such natural groups are not to be broken up, the method of dichotomy cannot be employed, for it necessarily involves such breaking up and dislocation. The group of [20] the Many-footed, for instance, would have some of its kinds distributed among land animals, others among water animals.
3 · Again, privative terms inevitably form one branch of dichotomous division, as we see in the proposed dichotomies. But privative terms in their character of privatives admit of no subdivision. For there can be no specific forms of a negation, of Featherless for instance or of Footless, as there are of Feathered and [25] of Footed. Yet a generic differentia must be subdivisible; for otherwise what is there that makes it generic rather than specific? There are to be found generic, that is specifically subdivisible, differentiae; Feathered for instance and Footed. For feathers are divisible into Barbed and Unbarbed, and feet into Manycleft, and Twocleft, like those of animals with bifid hoofs, and Uncleft or Undivided, like [30] those of animals with solid hoofs. Now even with differentiae capable of this specific subdivision it is difficult enough so to make the classification that each animal shall be comprehended in some one subdivision and in not more than one (e.g. winged and wingless; for some are both—e.g. ants, glowworms, and some others); but far more [35] difficult, impossible, is it to do this, if we start with a dichotomy into two contradictories. For each differentia must be presented by some species. There must [643a1] be some species, therefore, under the privative heading. Now specifically distinct animals cannot present in their substance a common undifferentiated element, but any apparently common element must really be differentiated. (Bird and Man for instance are both Two-footed, but their two-footedness is diverse and differentiated. And if they are sanguineous they must have some difference in their blood, if their [5] blood is part of their substance.) From this it follows that one differentia will belong to two species; and if that is so, it is plain that a privative cannot be a differentia.
Again, if the species are indivisible and the differentiae are indivisible, and if no differentia be common to several groups, the number of differentiae must be equal to the number of species. If a differentia though not divisible could yet be common to several groups, then it is plain that in virtue of that common differentia [10] specifically distinct animals would fall into the same division. It is necessary then, if the differentiae, under which are ranged all the indivisible groups, are specific characters, that none of them shall be common; for otherwise, as already said, specifically distinct animals will come into one and the same division. But no one indivisible group must be included in more than a single division; different groups must not be included in the same division; and every group must be found in some [15] division. It is plain then that we cannot get at the indivisible species of the animal, or any other, kingdom by bifurcate division. If we could, the number of ultimate differentiae would equal the number of indivisible animal species. For assume an [20] order of beings whose prime differentiae are White and Non-white. Each of these branches will bifurcate, and their branches again, and so on till we reach the differentiae, whose number will be four or some other power of two, and will also be the number of the ultimate species.
(A species is constituted by the combination of differentia and matter. For no part of an animal is purely material or purely immaterial; nor can a body, [25] independently of its condition, constitute an animal or any of its parts, as has repeatedly been observed.)
Further, the differen
tiae must be elements of the substance, and not merely essential attributes. Thus if Figure is the term to be divided, it must not be divided into figures whose angles are equal to two right angles, and figures whose angles are together greater than two right angles. For it is only an attribute of a triangle that [30] its angles are equal to two right angles.
Again, the bifurcations must be opposites, for opposites are different from one another—e.g. White and Black, Straight and Bent; and if we characterize one branch by either term, we must characterize the other by its opposite, and not, for example, characterize one branch by a colour, the other by an inclination. [35]