VII. SELECTIONS FROM ARISTOTLE

Physics and On Parts of Animals

Prepared by J. McAlhany & B. Schwartz

Physics Book 1 1

The scope and method of this book

Since in every field of inquiry scientific knowledge comes from recognizing those things that have principles, causes, or elements, it is clear that to attain scientific knowledge of "nature" we must first attempt to distinguish what concerns its principles. This is because we claim to know a thing when we discern its first causes and first principles, down to its elements.

The path to what is clearer and more knowable by nature starts from what is clearer and more knowable to us, for what can be known relative to us and what can be known absolutely are not the same. And so, our first way of proceeding leads by necessity from what is less clear by nature and clearer to us towards what is clearer and more knowable by nature. At first, things that are jumbled all together are clear and distinct to us, but later, its elements and principles become recognizable as we distinguish them. Thus, we must proceed from generalities to particulars, because the whole is better known by sense-perception, and a generality, since it contains many parts, is a kind of whole. It is much the same with a name in relation to its definition. A name, such as "circle," signifies something whole and indivisible, but its definition distinguishes it into its individual parts. For example, a child at first calls all men "father" and all women "mother," but later distinguishes its parents from all other men and women.

7

The number and nature of the principles

So let us begin to speak first about every kind of "coming to be," since it is correct by nature to investigate particulars individually by first speaking in general.

Whether we are talking about simple things or complex things, we say that one thing "comes from to be" from another, and some other thing "comes to be" from some other. For example, a "person becomes educated," or "uneducated becomes educated," or "an uneducated person becomes an educated person." In the first two examples, I call both the thing that becomes something (person or uneducated) and what the thing becomes (educated) "simple," but in the third example (an uneducated person becomes an educated person), both what becomes something and what it becomes are "complex."

In the simple type, something is said not only to become something but to become somethingfrom something (for example, to become educated from being uneducated), but this is not always the case with the complex type: we do not say "he became educated from being a person," but "the person became educated."

When things "come to be" in the sense we say that simple things do, in some cases there is a thing that continues to exist, but in other cases there is not. For example, when a "person becomes educated," the person remains and continues to exist as a person, whereas "uneducated" does not continue to exist, either on its own or in combination with something else.

Once we distinguish these types of change, we are able understand that if one considers it in the way we are describing, something must always continue to exist-the thing that becomes something-and though it may be one in number, it is not one in form (here, I call "form" and "definition" the same thing). For "to be a person" and "to be uneducated" are not the same thing; one continues to exist through the change, the other does not. Something that does not have an opposite continues to exist, as the person continues to exist, but "uneducated" does not, nor does anything composed of two elements continue to exist (such as "uneducated person").

In the case of things that do not continue to exist, we say that "A comes to be from B" instead of "B becomes A." For example, we say "educated comes to be from uneducated," not "educated comes to be from the person." And yet, sometimes even in the case of things that do continue to exist, we say "A comes to befrom B," as when we say "the statue comes to be from the bronze," not "the bronze becomes a statue." However, we say it both ways in the case of things that are in opposition and do not continue to exist, both "A comes to be from B" and "B becomes A." For example, we say both that "educated comes to be from uneducated" and "uneducated becomes educated." And we also talk the same way in the case of complex things: "the educated person comes to be from an uneducated person" and "the uneducated person becomes an educated person."

"Coming to be" has many different senses: some things are said not simply "to become," but "to becomesomething," while only substances are simply said "to become." In other cases, what comes to be must continue to exist by necessity. When something continues to exist, it becomes a certain size, or becomes a certain kind, or comes to be in relation to something else, because only substance is said to exist without any underlying relation to something else. Everything except substance exists in some relation to substance.

It should be clear to anyone who looks at the matter closely that substances (that is, everything that simply exists) come to be from some underlying thing. For there is always something that underlies them (called a "substratum"), and it is out of this substratum that whatever comes to be, comes to be (even plants and animals come to be from a seed). Of things that come to be simply, some come to be by changing shape (like a bronze statue), others by addition (things that grow), other by subtraction (a figure of Hermes carved from stone), others by combination (a house), others by alteration (things that are changed in their material). Clearly, everything that changes in these ways comes to be from something that continues to exist. So it is clear from what we have said that everything that comes to be is always complex: there is both something that comes to be (namely, the substratum) and the thing that it becomes. The thing that it becomes is one of two types: either it continues to exist or it is an opposite. For example, in the case of the uneducated person becoming educated, the opposite is the uneducated, and that which continues to exist is the person. Other kinds of opposite are shapelessness or disorder, while the bronze or stone or gold is the substratum.

So this is clear: since things that exist by nature have causes and principles, and since these things first exist from these causes and principles (that is, they have come to be not by chance, but in accordance with their substance), then everything comes to be from both substratum and form. So an educated person is compounded in some way from "person" and "educated," since you can break it down into the definitions of each element. Thus, what comes to be would come to be from these two things, substratum and form.

The substratum is one in number, but two in form (for "person" or "gold" or any material in general is what we are counting, for the material is more of a "thing"). And it is not by some accident, such as subtraction or opposition, that what these things become (for example, educated, or a statue) comes to be from them. There is one form that a thing takes on, such as the arrangement or the education or anything else so categorized. Therefore, one could say that there are either two or three principles: it is possible the principles could be two opposites (if someone, for example, should say educated and uneducated, or hot and cold, or harmonious and dissonant), or they could not be opposites, since opposites cannot act upon one another. But this problem is removed by the fact that the substratum is a third thing, since it is not an opposite. So in one way the principles are not more in number than opposites (that is, there are two), but neither are there exactly two, since their actual existence is a different thing, and so there are, in another manner of speaking, three. For "being a person" is different from "being uneducated," and "being shapeless" is different from "being bronze."

In regards to becoming and change, we have discussed the number of principles of things that exist by nature, and why they are that number. And it is clear that there must be something underlying the opposites (namely, the substratum) and that the opposites are two in number (though in another way this is not necessarily the case: it is sufficient for one of the opposites to cause a change by its presence or absence).

The underlying nature can be known by analogy: just as bronze is related to a statue or wood is related to a bed or what is shapeless is related, before it takes shape, to anything that has shape, so is the underlying nature related to substance (substance is what "this" is, what actually exists). This is one principle (not one or existing in the way the thing that "this" is exists), and one for which there is a definition. (There is moreover its opposite, absence, but in what way there are two principles, and in what way there are three, has already been stated.) It was first said that only opposites are principles, then later it was necessary that there be something else that continues to exist (the substratum), and so there were three principles. From this it is clear what the difference in opposites is, how they are related to one another, and what the substratum is. It is not yet clear whether form or the substratum is the essential being. But that the principles are three in number, how they are three, and in what way, is clear. From this let it be considered demonstrated how many principles there are and what they are.

Book 2 1. Nature and the natural

Of the things that exist, some exist by nature, others exist from other causes. Examples of things that exist by nature are: animals and their parts, plants, and simple bodies (earth, fire, air, water). For we say that these things, and things like them, exist by nature, and all these things are clearly different from things not constituted by nature. First, each of these has a principle of motion and rest in itself (some in respect to location, others in respect to growth and decay, others in respect to alteration). Yet a bed, or a coat, or anything else of this sort, insofar as it exists as a product of art or skill and has acquired a definite name, has no innate impulse to change. However, insofar as these things happen to be made of stone or earth or some mixture of these, they do have an impulse to change, but only to the extent thatnature is a principle and cause of movement and rest in whatever it is that exists primarily on its own and not as some accidental characteristic.

An "accidental characteristic" is like the way a person who is a doctor would be a cause of health for himself. This person does not possess medical knowledge insofar as he is a patient; it just happens to be the case by some accident that the same person is both doctor and patient. For these two things-"having medical knowledge" and "being a patient"-do not always occur together.

It is the same with everything that is made, since none of these has a principle of making in itself; rather, in some cases the principle is in things that are external to it (for example, houses and other things built by hand), while in other cases the principle is in the things themselves, but not according to themselves (for example, anything that would be a cause to itself by some accidental characteristic).

What we have described is "nature". For everything that has such a principle as we have described has a "nature," and all these things are "substances," for it is something that continues to exist, and "nature" always resides in something that continues to exist. These things, and everything which exists in them in virtue of what they are, exist "according to nature." For example, it is "according to the nature" of fire to move upwards, but this movement is not "nature," nor does this movement "have a nature," but it exists "according to nature" and "by nature."

Thus we have described what nature is, and what "by nature" and "according to nature" mean. It would be ridiculous to try to prove that nature exists, since it is obvious that there are many things like this. Only a person who cannot distinguish what is clear from what is unclear would try to distinguish what is self-evident from what is not (and clearly this happens sometimes, as when someone blind from birth reasons about colors, even though the blind person is only reasoning with the names of the colors and has no idea of what they really are).

Some people think that nature and substance are the primary thing, formless in itself, inherently existing in each thing that exists by nature, as wood is the "nature" of a bed and bronze the "nature" of a statue. As proof of this claim, Antiphon says that if someone buried a bed in the ground, and when the bed rotted it acquired the ability to send forth shoots, wood, and not a bed, would be produced. He says this is because the arrangement of the wood into the shape of a bed and the skill in crafting it exist only as accidental characteristics, but the thing that continues to exist even while the bed continues to change (that is, the wood) is the substance.

But if each of the things that exists by nature underwent the same process in relationship to something else, say bronze or gold in relation to water, or bones and wood in relation to earth, and so on, this "something else" would, by this argument, be their nature and essence. And so some people say that the nature of extant things is fire, some say it is earth, other say it is water, and others air; some people say the nature of extant things is some of these, while others claim it is all of these. Whatever someone assumes to be "nature" is thus claimed to constitute substance, and everything else is a change in state or some arrangement of this "nature" (and this is eternal, as everything that is not "nature" has no principle of change in itself). These people say that everything else comes to be and disappears again and again. So this is one definition of nature: the primary underlying material of each thing that has a principle of motion and change in itself.

But in another way, "nature" is the shape or form according to the definition of a thing.

For just as "craft" has two senses-namely, "according to craft" and "a thing crafted"-so too does nature have two senses: what is according to nature and what is natural. We would not say that if a bed exists only potentially and does not yet have the form of a bed that it has anything "according to craft", nor would we say that it has been crafted. Nor would we say this of something put together by nature. So what is flesh and bone potentially does not yet possess its own nature, nor does it exist by nature, until it acquires the form specified in the definition by which we call something "flesh" or "bone." And so, in this different sense, nature would be the shape or form of a thing, separable from the thing only in name, possessing the principle of motion within itself. What exists of these things is not "nature," but exists "by nature" (a human being, for example) Thus, form rather than matter is nature, since each thing is really said to exist when it exists in its complete and final form rather than when it exists only potentially.

So if matter is nature, then form is also nature, for a human being comes to be from a human being, but a bed does not come to be from a bed. And moreover, nature in the sense of "becoming" is a way to the nature of a thing. It is not a way like "practicing medicine", which leads not to the art of medicine but to health, since practicing medicine must come from the art of medicine, not lead to it. Nature in the sense of "becoming" is not like this in relation to a thing's "nature"; rather, what grows naturally, insofar as it grows naturally, leads from something to something. What then is "growing naturally"? Not from something, but to something. And in this way form is nature. But form and nature have two different senses, for deprivation is in some way a form. But whether or not deprivation-something contrary to becoming-exists in regards to simple coming to be must be examined later.

7.

Natural scientists should use the four conditions of change in their explanations

It is clear that there are causes, and there are, as we said, four of them, since there are as many causes as there are answers to the question "Why?" The question "Why?" ultimately leads to one of four causes:

1. In the case of things without motion, it leads to the question "What?" For example, in mathematics, the question ultimately ends up in the definition of straight line, symmetry, etc. (theformal cause)

2. Or it leads to what first brought about motion. For example: Why did they go to war? Because they had been attacked. (theefficient cause)

3. Or it leads to the question "for what purpose?" For example: Why did they go to war? In order that they may rule. (thefinal cause)

4. Or, in the case of things that come to be, it leads to the matter. (thematerial cause)

So it is clear that these are the four causes. Anyone interested in the natural world should know about all of them, and will trace in a scientific manner the question "why?" back to all these causes: matter, form, mover, purpose. The last three often come down to one, since "What is it?" and "For what purpose is it?" are really one and the same question, and "From where did the motion first come?" is the same in form as those two, since a human being begets a human being. And generally everything that is in motion causes motion, and everything that does not cause motion is not a part of natural philosophy, since, being without motion, they do not cause motion or have a principle of motion in themselves, but are rather incapable of motion. So there are three areas of inquiry: things without motion, things in motion that are indestructible, and things that are destructible.

Thus, the question "Why?" is referred back to matter, to the question "What is it?" (the form), and to what first brings about motion. It is in this way most of all that people investigate the causes of becoming: "What come to be after what?", "What first brought something about?", or "What first was acted on?", and so on in order.

The principles of natural motion are two, one of which is not part of natural philosophy, since it does not have a principle of motion in itself. An example of this is something that causes motion without being in motion itself, such as what is completely without motion and the most fundamental thing of all-the "What is it?", namely, the form. For this is the end and purpose, so that, since nature is for the sake of something, it is necessary to know what this is, and we must fully answer the question "Why?": for example, to know that A necessarily comes from B (either in every case or in most cases), to know whether A will come to be (as a logical conclusion from certain premises), and to know that this is what it is to be something, and that it is better to be this way, not absolutely, but in relation to the substance of each thing.

8.

We must discuss first why nature is one of the final causes, then discuss how natural scientists conceive of necessity, since they all explain things by this cause (they say, for example, that because hot and cold and everything else of this type is such and such a thing, then other things necessarily exist and come to be). And yet, if they should speak of some other cause, such as "love and strife" or "mind," they only briefly touch upon it and then forget about it. However, there is a problem: what prevents nature from creating something not for some purpose and not because it is better, but out of necessity? Just like when Zeus sends rain, it is not in order that crops grow, but simply it is necessary that moisture, when it rises, to grow cold, and when it grows cold, to fall to earth as rain; the crops just happen to grow when it rains. Likewise, too, if somebody's crops are ruined on the threshing-floor by a heavy rain, the rain did not fall to ruin the crops, but this just happens as a result. And so what prevents it being the same with parts in nature-teeth, for example grow by necessity: the front teeth are sharp and suitable for tearing, while the molars are flat and useful for grinding food-since they came to be not for this purpose, but just turned out this way? It's the same with other parts of the body, in however many there seems to exist some final purpose. And so wherever things turn out just as if they were for some purpose, these things survive, having been suitably formed spontaneously. And whatever did not turn out like this (like Empedocles' man-faced offspring of a cow) perished and continues to perish.

And this is the reasoning at which someone would be puzzled, even if it is slightly different. But it is impossible for the argument to hold true in this way. For these things and everything that exists by nature comes to be in certain way, either always or almost always, and none of them come to be by chance or spontaneously. For example, it often seems to rain during winter, and not by chance or coincidence, but when the dog-star is in the sky in summer, it seems to rain by chance. Nor are hot spells considered to occur by chance in the summer, but during the winter, they are. If then things seem to be either the result of a coincidence or for some purpose, and if it is not possible for these things to exists either by coincidence or spontaneously, they would have to be for some purpose. And indeed all such things exist by nature, as even those making the argument above would have to confess. Thus, a final cause-purpose for which something is-is in things which come to be and exist by nature.

Book 3 1. The nature of change

Since nature is a principle of motion and change, and since our inquiry is about nature, we must not overlook the question of what motion is. For without understanding motion, we could not understand nature. Once we define the parts of motion, we must attempt to approach in order the things concerning motion in the same way.

Motion seems to be something continuous, and what is first apparent in the continuous is the infinite. And so it often happens that people who are defining the continuous make use of the definition of the infinite, as in "What is infinitely divisible is continuous." In addition to these, it is thought that it is impossible for motion to exist without place, void, and time.

It is clear, then, that since for these reasons, and because these things are common to all things and universal, we must make our inquiry by considering each of these items (the investigation of the particulars follows the investigation of the generalities). First, as we said, we will start with motion.

There are things that only exist actually and things that exist potentially and actually: one is a "this", another is "this much", another is "this sort", and likewise with all the other categories of extant things. The term "in relation to" is used, on the one hand, in regards to excess and to lack, and on the other, in regards to what acts and what is acted on (and generally what can cause motion and what can be moved). For what can cause motion is the mover of what can be moved, and what can be moved is moveable by what can cause motion. But there is no movement without the things themselves. For whatever changes, changes in substance or in quantity or in quality or in location, but it impossible, as we claim, to find anything common to these which is not a "this thing" or some quantity or some quality or some other characteristic. Thus, there will be neither motion nor change without the categories just mentioned, since there is nothing that exists without them.

Each of these categories exists in all things in two ways: for example, what the thing is, is either form or its absence. Quality could be, for example, either black or white, quantity could be either complete or incomplete. It is the same with movement: up or down, light or heavy. And so there are as many forms of motion and change as there are ways of being.

Having distinguished in each case between what is actual and what is potential, we say that the actualization of what exists potentially, insofar as it exists potentially, is motion. For example, the alteration of something that can be altered, insofar as it can be altered, or the increase or decrease (there is not a single word covering both) of what can increase or decrease, or the coming-to-be and passing away of what can come to be and pass away, or the movement of what can be moved.

That this is motion will be clear from the following. Whenever something buildable, insofar as it is buildable, actually exists, it is being built, and this is the act of building. Likewise with learning, practicing medicine, rolling, jumping, ripening, and aging. Since some things can exist both potentially and actually (but not at the same time nor in the same respect: for example, actually hot but potentially cold), many things will act on and be acted on by one another, because everything will be at the same time capable of acting on something else and capable of being acted upon. Thus, what naturally causes motion can be put in motion, for everything of this kind, when it is put in motion, also moves itself. Some people think that everything that causes motion is also moved. (However, this will be made clear from other arguments, since there is something that causes motion yet is immovable.)

But motion is the fulfillment of what exists potentially, when it exists in actual fulfillment-not insofar as it is itself, but insofar as it is moveable.

By "insofar as" I mean the following: bronze is potentially a statue, but nevertheless, it is not the actualization of the bronze, insofar as it is bronze, that is motion, because it is not the same thing to exist as bronze and to exist potentially. If they were the same thing without qualification and in definition, the actualization of the bronze, insofar as it is bronze, would be motion. But they are not the same, as was said. This is clear in the case of opposites: "to be capable of health" and "to be capable of sickness" are different; otherwise "being healthy" and "being sick" would be the same thing. But what is underlying, the thing that is healthy or the thing that is sick, whether it be blood or some bodily humour, are one and the same thing. But since "to exist as bronze" and "to exist potentially as a statue" are not the same, just as "to be a color" is not the same as "to be visible," it is clear that motion is the actualization of something potential, insofar as it is potential.

Furthermore, that this is motion, and that it there happens to be motion just when the actualization occurs, not before or after, is clear. For each thing is sometimes able to become actual, but other times not. For example, in the case of something buildable: the actualization of its "buildability," insofar as the thing is buildable, is the act of building. For the actualization of the buildable thing must be either the act of building or the house. But if the actualization were the house, at that point the buildable thing is no longer buildable, because the buildable has then been built. Thus, the actualization has to be the act of building, and the act of building is a kind of motion. And the same account will also apply in the case of other types of motion…

On the Parts of Animals

Book 1 1

In every study and inquiry, whether lowly or noble, there seem to be two kinds of skill: one can well be called "scientific knowledge" of a subject, while the other is a kind of general education. A generally educated person can in some way accurately judge whether a speaker explains something well or not, and we consider a person who is generally educated to be such a person, and to be educated is to be able to do what we just said. However, we consider only a single individual who is a judge of sorts in just about all areas to be generally educated, while someone else could be a judge of only one particular subject. For a person could be proficient in the same way as we described in only a part of a subject.

And so it is clear that also in the study of nature there must be some such standard definitions which someone can use to understand the method of an explanation, regardless of whether the statements are true or false. Here's an example of what I mean: should we take each single substance and treat it separately on its own (as, for example, taking the nature of a human, or a lion, or an ox, and so on, each individually), or should we begin by proposing some general characteristics common to all of them? There are many things that are shared among different kinds of living beings, such as sleep, respiration, growth, decay, and death (in addition to these, there are many things among the affects and states of living beings which we will pass over, since it is not now possible to discuss them clearly and distinctly). And it is clear that in speaking about many different things individually we will often say the same things over and over. For each of the things we just listed (sleep, respiration, and so on) belongs to horses and dogs and humans, so that if someone speaks about each of their characteristics in turn, he will frequently be compelled to speak about the same things, to the extent that the same things, not at all different themselves, belong to animals that are different in form. There are perhaps other characteristics which happen to fall under the same category, though they differ in respect to form. For example, the movement of animals does not appear in one form only, since flying, swimming, walking, and creeping are different. Therefore, we must not overlook how we are to proceed in our investigation. What I mean is, should we start with what is common to each species, and then look into the particulars, or immediately begin with each individual type? There is at present no rule about this, not even about the following. That is, should the physicist to proceed like mathematicians when they lecture about astronomy, namely, to first investigate the phenomena concerning living things and their individual parts, and then to discuss the "why" and the causes?

In addition to this question, since we see that there is more than one cause related to natural generation (such as the purpose, and where the principle of motion come from), we must also decide which of these causes was first, and which came into being second. The first cause is plainly the one we call the "final" cause, the "for the sake of which." For this is the reason, and the reason is the first principle in both things constituted by art and things constituted by nature. Both the doctor, by defining health either in thought or in sense-perception, and the house-builder, by defining a house in thought or in sense-perception, provide the reasons and causes of what they do at each step, and why their actions must be done in a particular way. However, the final cause and the good is found in works of nature more than in the works of art. But necessity does not belong to all things according to nature in the same way, though nearly everyone tries to base their arguments on it, without distinguishing the number of ways "necessity" is meant. "Absolute necessity" belongs to eternal things, but "hypothetical necessity" belongs also to all things generated by nature as well as in things made by art (such as a house and anything else of this sort). For it is necessary for some kind of material to exist, if a house or some other end product is going to exist, and it is also necessary that first one thing come to be and be set in motion, then another thing, and so on in a series until the final end, for which each thing comes to be and exists, is reached. It is the same way with things generated by nature.

The manner of demonstration and the type of necessity are different in the case of natural science from that of theoretical knowledge, and I have spoken about this in other works. The starting point for the theoretical sciences is whatis, and the starting point for the natural sciences is whatis to be. Since "health" or "human being" is somekind of thing, it is necessary that something be or come to be, but it is not the case that since some one thing exists or has come into being, health or a human necessarily exists or will exist. Nor is it possible for the necessity of such a demonstration to extend in a series for eternity, so that one could say "Since A exists, B exists." This too has been discussed in a different work, namely, to what sort of thing necessity belongs, what sort of things it changes, and for what reason.

We also must not forget to ask whether it is fitting to ask, just as earlier thinkers did, how each thing tends to come into being rather than how it exists. For there is no small difference between the two. It seems reasonable to begin in accordance with what we said above, to begin first with the phenomena for each species, then discuss their causes, and then generation. This is also what happens with house-building. The form of a house is such-and-such a thing, or a house is such-and-such a thing, because it came to be in a particular way. For generation is for the sake of substance, but the substance is not for the sake of generation. This is why Empedocles did not speak correctly when he said that animals possess many characteristics simply because they happened to be this way in their generation. For example, he said that an animal has a certain kind of backbone, because it just happened by accident that the animal's backbone was broken as it was twisting in the womb. First of all, he did not realize that the seed which brings the animal together must already exist with such a capability, and secondly, he did not realize that the animal which was produced first existed not only in thought, but also in time. For a human begets a human. Because the parent is a particular kind of thing, the generation of the offspring happens to be of a particular sort. It is the same with things that seem to come into being spontaneously, as in the case of products of art. For the thing that comes into being spontaneously can also be created from art (health, for example). In some of these cases, however, the creative force pre-exists the things created (such as with making statues, since they do not come into being spontaneously). The art is the plan of the work which exists without material. Things that come into being by chance act in the same way as those from art, for they come into being in the same way as art.

Therefore, it must be said most of all that, since the essence of being a human is such-and-such a thing, a human has certain parts on account of being what it is. For we cannot admit that a human exists without these parts. Otherwise, we can say what is the closest thing to this, that either it is generally the case that a human being must have these parts (since it is impossible for a human to exist otherwise) or at least we can say that it is good that a human exists in this way. And this is the logical consequence: since a thing is a certain kind of thing, it is necessary that its generation occur in a particular way and be of a certain kind. And thus, first one of its parts comes into being, and then another. And this is how it happens in all things put together by nature.

Earlier writers and the first natural philosophers investigated the material principle and the causes of it being what it is, and what sort of thing it is, and how everything came to be from it, and what was the mover (for example, strife or love or mind or spontaneous action). They assumed that the underlying matter has by necessity some such nature, for example, the heat of fire, or the cold of the earth, the lightness of one, or the weight of the other. In this way, they explain the generation of the universe, and they talk in the same way about the generation of animals and plants. For example, they say that the flow of water in the body creates an empty space for every receptacle of food and excretion, or that the passage of the breath breaks open the nostrils. And so they say air and water are the material of bodies, since they all propose nature as being composed from such bodies. But if humans and animals and their parts exist by nature, one would have to talk about the flesh and the bone and blood and all the homogenous parts, and also about the heterogeneous parts, such as the face, the hand, and the foot, and explain how each of them is such a thing and by virtue of what force. For it is not enough to say that they exist from something, such as fire or earth, no more than it would be enough to explain a bed or some such thing by saying that it was made of bronze or wood, without trying to describe its form or its matter, or at least the sum of its parts. For a bed is such-and-such a form in such-and-such matter, or is a certain kind of thing, so that even if we would have to speak about its shape, we would also describe what sort of form it has. For nature in respect to form is more important than the material nature.

If then each of the animals and their parts possess shape and color, Democritus spoke correctly, since he appears to have assumed this. He says that it is obvious that a human exists in some sort of form, since a human is recognizable by its shape and color. And yet a dead human has the same shape as a living human, but nevertheless, it is not really a human. And furthermore, it is impossible that a hand could be constituted in any way whatsoever, such as out of bronze or wood; these are hands in name only, just as a picture of a doctor is not a doctor except in name. For a hand of wood or bronze will not be able to perform the function of a hand, just as a stone flute or a picture of a doctor could not perform their functions. In the same way as this, the parts of a dead body, such as an eye or a hand, are not really an eye or a hand. So Democritus spoke too simply, in the same way as if a woodworker talked about a wooden hand as a real hand. This is the same way that some natural scientists talk about the generation and causes of form, because these forms have been fashioned by some forces. But perhaps the woodcarver will say those forces are an axe or an auger, while another will say air and earth-but the woodcarver makes more sense. And yet it is not sufficient for him to say only that a hole comes into being here, and a flat surface there, because his tool made a cut here or there. Rather, he must say why he made this cut, and for what reason-then he will give the cause of how the wood came to be this shape or that. It is clear then that the thinkers who talk in this way speak incorrectly, and that one must explain that an animal is such-and-such a thing, as well as explain what it is and what sort of thing it is, and also explain each of its parts, just like when discussing the form of a bed.

Indeed, if the thing that constitutes the form of a living being is a soul or a part of the soul, or something that cannot exist without a soul (for if the soul departs, there is no longer a living being, and none of the parts remain the same except the shape, like animals turned into stone), then the natural scientist has to discuss and know the soul, if not in its entirety, at least in respect to that part which makes a living being a living being. The natural scientist also has to explain what the soul is or what this part is, and about all the characteristics which make it what it is, especially since nature is spoken of in two senses, being either the matter or the essence (which is the mover and the final end). And every soul or part of a soul of a living being belongs to the second of these. Thus, someone investigating nature must speak about the soul rather than the matter, to the extent that the matter is nature because of soul rather than the other way around. Indeed, wood is a bed or a tripod only because it is these things potentially.

Considering what we just said, someone could raise the question of whether it is in the realm of natural science to speak about the whole soul or only some part of it. If it is the whole soul, there is no philosophy except knowledge of the natural world, since the intellect itself would be one of the objects of study. And thus natural science would be knowledge of everything, for it would belong to the same science to investigate both intelligence itself and the objects of intelligence (if in fact they are related to one another), and the same single science would concern everything that is related, such as both perception and the objects perceived. But perhaps it is not the whole soul nor all its parts that is the principle of motion, but rather only one part, such as in plants, which is a principle of growth, or the sensory part, which is a principle of alteration, or some other part, but not the intelligent part, which is a principle of movement. For locomotion exists in animals other than humans, but not intellect. It is clear then that we need not speak of the whole soul, for it is not the whole soul which is nature, but either some one or more parts of it.

Furthermore, there cannot be any natural science of abstract things, since nature does everything for the sake of something. For just as the artist's skill is apparent in works of art, so too is there some other such principle or cause manifest in living things themselves, derived, like hot and cold, from the entire universe. And so it is more likely that the heavens came to be from some such cause, if they did in fact come to be, and that they exist on account of some such cause more than mortal beings. For arrangement and division are much more apparent in heavenly bodies than in us, and chance and randomness are much more apparent in mortal beings.

Some people say that every living being came to be and exists by nature, but that the heavens were formed as they are by chance and spontaneously-but nothing whatsoever in the heavens appears to be from chance or disorder. We always say that something exists for the sake of something, wherever there is clearly some end to which motion tends as long as nothing stands in its way. And so it is clear that there is some such thing which in fact we call nature. For not any random thing comes to be from any random seed, but a particular thing comes from a particular seed, nor does any random seed come from any random body. And so the seed is a principle and maker of the thing from which it comes, for it is these things by nature. And yet the thing from which the seed came existed prior to the thing generated, for the seed is the coming-to-be, and the result is the being. And yet prior to these two things is the thing from which the seed originated. For a seed is spoken of in two senses, that from which it came and that which it will become. So there is the thing from which the seed came (for example, the seed of a horse) and the thing which will exist from the seed (for example, a mule), but these are not seeds in the same way. And there is furthermore a seed potentially, and the relation of potential to actual we already know.

These then are the two causes, purpose and necessity. For many things come to be simply out of necessity. Someone could perhaps ask what sort of necessity is meant by "out of necessity." For it cannot be either of the two ways set forth in philosophical works. There is a third kind of necessity in the case of things that have a generation, for we say that nourishment is something necessary not in either of the two ways, but because it is not possible for living things to exist without it. This necessity is "hypothetical necessity." For example, if one has to cut wood with an axe, it is necessary that the axe be hard, and if the axe is to be hard, it is necessary that it be made of bronze or iron. In the same way, since the body is an instrument of sorts (because each of its parts and the body as a whole are for the sake of something), it is thus necessary that it be of such-and-such a character and be made of such-and-such things, if it is going to be a kind of instrument.

It is clear then that there are two types of cause, and that we must take both into account in our discussion, or at least try to; it is also clear that those who do not discuss both of them essentially say nothing about nature, since nature is more of a principle than matter is. Empedocles, led along by truth, hits upon this idea somewhere in his work, when he is compelled to claim that proportion is the essence and nature of things, as when he explains what bone is. He does not just say that bone is one or two or three of the elements, but he gives the proportions of their mixture. Flesh, of course, exists in the same way, as well as each of the other body parts. The reason that previous thinkers did not come upon this method is that they did not have any notion of essence ("what it is to be something") or any definition of substance. Democritus first came close to the idea, not as something necessary for natural science, but he was led along to it by the facts. This idea was developed further in Socrates' day, but the investigation into the natural world was abandoned, and philosophers turned their attention to practical topics such as ethics and politics.

Here's an example how our demonstration will proceed: we would say that respiration exists for some purpose, and it comes about on account of certain things by necessity. Necessity sometimes means that if some one thing is going to be the purpose, then it is necessary to possess some other things (hypothetical necessity), and sometimes it means that things are and naturally came to be in a certain way (absolute necessity). So in order for humans to live, it is necessary that heat depart and reenter in alternation and for air to flow in. And this indeed is a necessity in the hypothetical sense. But when the collision of the warm air and the cold by necessity produces an inflow and outflow of the outer air, this is a necessity in the absolute sense. This is the method of our inquiry, and these are the kinds of things whose causes we must investigate.