Reading Aristotle: Physics 2.1: What is Nature?

*Note: I said that before I began commentary on Book 2, I’d write an outline of Book 1. Unfortunately that project is taking longer than I expected. I still intend to complete it, but thought in the meantime I would go ahead and begin Book 2 anyways.

Having completed Book One of the Physics, in which Aristotle explored the fundamental principles of nature, we turn now to Book Two, which begins with asking what nature itself is:

“Of things that exist, some exist by nature, some from other causes. ‘By nature’ the animals and their parts exist, and the plants and the simple bodies (earth, fire, air, water)–for we say that these and the like exist ‘by nature’. All the things mentioned present a feature in which they differ from things which are not constituted by nature. Each of them has within itself a principle of motion and of stationariness (in respect of place, or of growth and decrease, or by way of alteration). On the other hand, a bed and a coat and anything else of that sort, qua receiving these designations — i. e. in so far as they are products of art — have no innate impulse to change. But in so far as they happen to be composed of stone or of earth or of a mixture of the two, they do have such an impulse, and just to that extent–which seems to indicate that nature is a source or cause of being moved and of being at rest in that to which it belongs primarily, in virtue of itself and not in virtue of a concomitant attribute” (Physics 2.1, 192b8-23) [1].

As Aquinas points out, Book One of the Physics was primarily directed towards the “principles of natural things”, whereas Book Two is primarily directed towards the “principles of natural science” itself (Lectio 1.141) [2]. To know the principles of any science, we must first know “its subject and the method by which it demonstrates” [3]. And the subject of natural science is, of course, nature; hence the discussion of the definition of nature.

Aristotle begins by noting a distinction among all that exists between things which “exist by nature” and things which exist “from other causes.” This is an obvious distinction which we still make use of today: is something natural or artificial? Something occurring “in nature” or something “man-made”? But what exactly does it mean for something to be natural in the first place? Aristotle lists a number of clear examples of the types of things which fit in this category: animals, plants, elements, to which we might potentially add planets, stars, atoms, particles, etc. These exist, he says, “by nature”, whatever that might turn out to entail.

And what do all those things have in common? They all “present a feature in which they differ from things which are not constituted by nature”, which feature is that “each of them has within itself a principle of motion and of stationariness”. In other words, natural things seem to have an “innate impulse to change”, which is intrinsic rather than extrinsic. A seed, for instance, will naturally (when the right conditions are met) turn into an oak tree; and the principle of this motion, its impetus, comes from within itself. A bed, on the other hand — what he calls “products of art” and what we today might call “artificial” — only receives its principle of motion extrinsically. A bed is something constructed out of other objects which ordinarily and naturally would not come about or arrange themselves into that particular configuration. If a bed is made entirely of wood, we can say with confidence that if we left a tree alone for a thousand years it would never on its on, as a result of anything about itself, transform into a bed. To make the wood of a tree into a bed is to make it do something it would never have done on its own, from its own natural tendencies/inclinations. To make a bed or other artifact is to impose form upon the matter of the wood.

As Aquinas explains, to say that something has a principle of motion within itself is not to say that it is not being moved by an external agent cause. In the example of the seed, for instance, the seed requires other factors outside itself in order to grow (sunlight, soil, water, etc.). But the seed itself has to have the ability to take in and use these external factors in the process of its own growth; the seed has to have the potency to receive these things and be moved by them. Thus for some things to say that they have a principle of motion within themselves is to say that they have within themselves the natural potency to be moved in certain ways. A seed is naturally directed, on its own, to take in water and sunlight in order to grow. But a tree is not naturally directed, on its own, to be configured into the shape of a bed. And so Aquinas comments that “For this reason the production of artificial things is not natural. For even though the material principle is in that which comes to be, it does not have a natural potency for such a form” (Lectio 1.144) [4]. But a bed really is composed of natural things as its material cause (at least, a wooden bed would be), and so insofar as it is composed of natural things, it has a principle of motion, but only to that limited and artificial extent. One way to think of this might be in terms of properties: wood has certain natural properties which it retains to an extent even when configured into a non-natural form. A bed has the properties of wood insofar as wood is its material cause.

So to be “natural” as opposed to artificial, to have a nature, is to have “a source or cause of being moved and of being at rest in that to which it belongs primarily, in virtue of itself and not in virtue of a concomitant [per accidens] attribute.” Simplified, we might say that to have a nature is to have intrinsic capacities and tendencies to move and to be moved in certain ways.

Aristotle continues:

“I say ‘not in virtue of a concomitant attribute’, because (for instance) a man who is a doctor might cure himself. Nevertheless it is not in so far as he is a patient that he possesses the art of medicine: it merely has happened that the same man is doctor and patient — and that is why these attributes are not always found together. So it is with all other artificial products. None of them has in itself the source of its own production. But while in some cases (for instance houses and the other products of manual labor) that principle is in something else external to the thing, in others — those which may cause a change in themselves in virtue of a concomitant attribute — it lies in the things themselves (but not in virtue of what they are)” (Physics 2.1, 192b23-32) [5].

In Aristotle’s definition of what a nature is, the “in virtue of itself and not in virtue of a concomitant attribute” part is quite significant; for it seems that there are some artificial/”man-made” objects which do have a certain kind of principle of motion within themselves (I think this is especially so today in the age of advanced technology, vehicles computers, robotics, etc.). The difference between these artificial objects and natural objects comes down, as this passage indicates, to whether the principle of motion within them is per se or per accidens (to use Aquinas’s language). The example of the doctor illustrates this. To be cured is a movement or change from sickness/injury to health. A person becomes cured only insofar as they were before sick; it makes no sense to speak of a completely healthy person being cured. And if we take the term “doctor” broadly to designate anything that cures, then something only cures insofar as it is a doctor. Now suppose that there is a doctor who is sick and who then cures himself. It is not insofar as he is a doctor that he becomes cured, it is insofar as he is sick; and it is not insofar as he is a patient that he cures, but insofar as he is a doctor. In other words, the doctor cures in virtue of being a doctor, and he is cured in virtue of being a patient; but it is entirely accidental that he happens to be in this scenario both doctor and patient. There is nothing intrinsic to being a doctor that requires one to also be a patient; and there is nothing intrinsic to being a patient that requires one to be a doctor. Most of the time, in fact, the roles are separated into distinct subjects.

Consider it this way: suppose there are two different people, A and B. B is sick, and A cures him. A is thus the per se principle of B’s health. But now suppose that there is just one person, C. C is a doctor and cures himself. He is thus the principle of his own health, but only per accidens. For the principle of his health is his being a doctor, not his being sick. In other words, C did not cure C insofar as he is himself, but only insofar as he is a doctor. And C did not become cured insofar as he is a doctor, but insofar as he was sick. We might say that the “nature” of being a doctor does not entail being sick, and the “nature” of being cured does not entail being a doctor.

And the same, Aristotle says, is true of all artificial things. Some artificial things have their principle of motion as obviously external to themselves, such as a house which is constructed by builders. But other artificial things appear to have the principle of motion within themselves, but it is only accidentally such, not “in virtue of what they are”. The metals which make up a clock may move, but it is not qua metals that they do so; they move only insofar as their natural potentials have been manipulated and a function imposed upon them.

“‘Nature’ then is what has been stated. Things ‘have a nature’ which have a principle of this kind. Each of them is a substance; for it is a subject, and nature always implies a subject in which it inheres” (2.1, 192b33-34) [6].

So nature is an intrinsic principle or source of motion in things in virtue of themselves, and things which have this principle have a nature. Possessing a nature is what separates a natural thing from an artificial one. Aquinas expands: “Nature is a subject insofar as it is called matter, and nature is in a subject insofar as it is called form” (Lectio 1.146) [7]. Each object that has a nature is a substance, and a substance is a composite of form and matter. The matter is a potency to possess the form, so it is the “subject” in which the nature inheres.

“The term ‘according to nature’ is applied to all these things and also to the attributes which belong to them in virtue of what they are, for instance the property of fire to be carried upwards — which is not a ‘nature’ nor ‘has a nature’ but is ‘by nature’ or ‘according to nature'” (2.1, 192b35-39) [8].

So we can say “according to nature” or “by nature” both of things which possess natures, and of certain attributes/properties which the nature entails. That fire is carried upwards is a property of its nature, but the “carried upwardsness” is not in itself a nature, but rather comes from a nature. Or we might say that it is “natural” for flame to be carried upwards and by this we would just mean that it belongs to the nature of fire to do so.

What nature is, then, and the meaning of the terms ‘by nature’ and ‘according to nature’, has been stated. That nature exists, it would be absurd to try to prove; for it is obvious that there are many things of this kind, and to prove what is obvious by what is not is the mark of a man who is unable to distinguish what is self-evident from what is not. (This state of mind is clearly possible. A man blind from birth might reason about colours. Presumably therefore such persons must be talking about words without any thought to correspond.)” (2.1, 193a1-8) [9].   

For Aristotle to here say that it is so abundantly obvious that natures exist that to even try to prove such is “absurd” might seem a bit surprising, but I think it is actually a more modest claim than it might initially come across. Remember that a nature here is just understood as a principle of motion within things; and so Aristotle is saying that it is evident to our senses that there are in facts things which have a principle of motion within themselves. In other words, we observe and experience with our sense things which move and which appear to move by virtue of something intrinsic about themselves. That we observe such things is clearly undeniable: animals, plants, seeds, etc. Above all else, we immediately and reflectively experience this reality in ourselves. Since this is self-evident fact to our senses, it would be useless to try to “prove” that it is so, in the same way that it is useful for people who can see to try to “prove” that color is real. It is not something that needs to be supported by proof, because it is directly known by experience. Aquinas writes:

“For it is manifest to the senses that many things are from nature, which have in themselves the principle of their own motion . . . The existence of nature is known per se, insofar as natural things are manifest to the senses. But what the nature of each thing is, or what the principle of its motion is, is not manifest” (Lectio 1.148) [10].

We know from experience that things have natures or intrinsic principles of motion, but this self-evident knowledge might not tell us exactly what that principle of motion is within the thing.

Aristotle goes on:

“Some identify the nature or substance of a natural object with that immediate constituent of it which taken by itself is without arrangement, e. g. the wood is the ‘nature’ of the bed, and the bronze the ‘nature’ of the statue. As an indication of this Antiphon points out that if you planted a bed and the rotting wood acquired the power of sending up a shoot, it would not be a bed that would come up, but wood — which shows that the arrangement in accordance with the rules of the art is merely an incidental attribute, whereas the real nature is the other, which, further, persists continuously through the process of making” (2.1, 193a9-17) [11].

The view discussed here is a kind of reductionism which identifies the nature of a thing with its material constitution. On this view, what we consider to be “natural” objects are essentially similar to what we consider to be artificial objects. As we’ve said, the “form” of a bed is an accidental form that is imposed extrinsically upon the material of the wood. The arrangement of the wood into the shape of a bed is “merely an incidental attribute”. What is natural about the bed is only that it is made up of what was a natural object, namely the wood which came from a tree. But on this reductive view, all things are ultimately like this. In other words, not just man-made artifacts, but man himself, and all animals and plants, are ultimately just accidental forms made up of some fundamental natural substances. So, on this view, humanity is to the atoms which makes up our bodies just what the shape of a bed is to wood; there is no actual “human nature” anymore than there is an actual “bed nature.” What we call humanity, or treeness, or aligatorness, is really just an incidental arrangement of prior material substances. Many of the pre-socratics thought that all the objects we perceive are ultimately reducible to a few basic material elements, and that these elements are the only real natural substances. So Aristotle goes on to say:

“But if the material of each of these objects has itself the same relation to something else, say bronze (or gold) to water, bones (or wood) to earth, and so on, that (they say) would be their nature and essence. Consequently some assert earth, others fire or air or water or some or all of these, to be the nature of the things that are. For whatever any of them supposed to have this character — whether one thing or more than one things — this or these he declared to be the whole of substance, all else beings its affections, states, or dispositions. Every such thing they held to be eternal (for it could not pass into anything else), but other things to come into being and cease to be times without number. This then is one account of ‘nature’, namely that it is the immediate material substratum of things which have in themselves a principle of motion or change” (2.1, 193a18-29) [12].

So, on this view, wood is to a bed as fundamental matter is to all objects. Nothing else has a nature or essence besides the fundamental material principle. This alone has its own source of motion intrinsic to itself essentially; everything else derives its principle of motion from this fundamental matter accidentally.

“Another account is that ‘nature’ is the shape or form which is specified in the definition of the thing. For the word ‘nature’ is applied to what is according to nature and the natural in the same way as ‘art’ is applied to what is artistic or a work of art. We should not say in the latter case that there is anything artistic about a thing, if it is a bed only potentially, not yet having the form of a bed; nor should we call it a work of art. The same is true of natural compounds. What is potentially flesh or bone has not yet its own ‘nature’, and does not exist ‘by nature’, until it receives the form specified in the definition, which we name in defining what flesh or bone is. Thus in the second sense of ‘nature’ it would be the shape or form (not separable except in statement) of things which have in themselves a source of motion. (The combination of the two, e. g. man, is not ‘nature’ but ‘by nature’ or natural.)” (2.1, 193a30-193b7) [13].

So one view of what nature consists of is that it is the material substratum of a thing. Another view is that it is the form of a thing. In explaining this, he again compares what we call natural to what we call artificial. A bed is an artifact insofar as it is configured by human design. But the wood by itself, before it is arranged into the bed, cannot properly be called an artifact, because it is at that point only potentially such. In the same way, suggests Aristotle, we cannot call the material substratum of natural objects the “nature” of the object, because without the form of the object, which establishes its definitive quality, the matter is only potentially that object. The definition of a human is a rational animal. The material components to a human are, for instance, its flesh, blood, bones, organs, etc. These things on their own are only in potentiality to what constitutes the definition of a human being. It is only when the form of humanness is present with the matter that the actual human being is there; and so on this view it is the form which is the nature. (And on this view, you would not say that the man as a whole is the nature, since the form itself is the the nature and the whole man is a compound of both form and matter, so it would be by nature or resulting from nature).

Which view does Aristotle take?

“The form indeed is ‘nature’ rather than the matter; for a thing is more properly said to be what it is when it has attained to fulfilment than when it exists potentially. Again man is born from man, but not bed from bed. That is why people say that the figure is not the nature of a bed, but the wood is — if the bed sprouted not a bed but wood would come up. But even if the figure is art, then on the same principle the shape of man is his nature. For man is born from man” (2.1, 193b7-12) [14].

A nature is that by which a thing is what it is; and a thing is most properly said to be what it is when it is actually what it is, not merely potentially. Form is that by which a thing is in actuality; hence, form is most properly said to be the nature of a thing, rather than matter, which is potentiality.

Here he offers another distinguishing feature between natural things and artificial things. An aspect of the principle of motion of a thing is its generation. Natural things are naturally generated from natural things. Antiphon’s point about the bed is to show that it has its shape only accidentally, and that it is not a natural substance but is made up of natural substance. This is evident from the fact that if one were to somehow be able to take the wood of a bed and plant it, and if it were somehow able to germinate, it would sprout into a tree, not another bed. The underlying point here is just that a tree is naturally directed towards the production of a tree, not towards the artificial configuration of a bed. So in the case of the bed, it really is correct to say that its nature is not in the shape but rather in the matter, because its shape is accidental, artificial, and externally imposed upon it via manipulation of the material. But if we conclude that bed is an artifact precisely because it is constituted of materials which are not naturally directed towards its form, then for this very reason we should conclude that man and other substances are natural, because they are naturally directed towards that form, as is evident from the fact that man gives birth to man.

“We also speak of a thing’s nature as being exhibited in the process of growth by which its nature is attained. The ‘nature’ in this sense is not like ‘doctoring’, which leads not to the art of doctoring but to health. Doctoring must start from the art, not lead to it. But it is not in this way that nature (in the one sense) is related to nature (in the other). What grows qua growing grows from something into something. Into what then does it grow? Not into that from which it arose but into that to which it tends. The shape then is nature” (2.1, 193b13-18) [15].

Here he says that we also see the nature of a thing displayed in its growth to fulfillment. The fulfillment is when the “nature is attained” or when it is fully actualized. But he differentiates this from a process such as doctoring. Doctoring is a process which does not end in doctoring, but in a patient coming to health or “having been doctored.” Aristotle thus sees this as an active principle acting upon a passive recipient (the sick patient). The “name” of the process — doctoring — derives from the active principle, the impetus rather than the end. But this is not the type of process he sees in what he refers to as “the process of growth by which a nature is attained”. This is a passive process, i.e. the nature is being received in what has the potency to receive it. In the process of “growth” by which this is accomplished, the thing undergoing the process “grows” from something into something. The process tends towards the receiving of the nature, which is what it grows into, not what it grows from. But in such a process, it is form that is received by matter; thus form is the nature of a thing, not matter.

This is all a bit abstract, but I think what Aristotle is getting at in speaking of nature exhibited in the process of growth is the idea that when something natural is generated/born, this process of generation leads to the nature, it is how the nature comes to be. But what is it that comes to be in the process? The form; and so the form is its nature.

He concludes Chapter One:

“‘Shape’ and ‘nature’, it should be added, are used in two senses. For the privation too is in a way form. But whether in an unqualified coming to be there is privation, i. e. a contrary to what comes to be, we must consider later” (2.1, 193b19-22) [16].

I will let Aquinas explain this one to end:

“The nature which is form is used in two ways, i.e., of the incomplete form and the complete form. This is clear in accidental generation, for example, when something becomes white. For whiteness is a complete form, and the privation of whiteness is in some way a species, insofar as it is joined to blackness, which is an imperfect form. But whether or not in simple generation, which is the generation of substances, there is something which is a privation and also a contrary, so that substantial forms are contraries, must be considered later” (Lectio 2.156) [17].




[1]. McKeon, Richard, editor. The Basic Works of Aristotle. New York: Random House, Inc, 1941. Print, 236.

[2]. Thomas Aquinas. Commentary on Aristotle’s Physics. Books I-II translated by Richard J. Blackwell, Richard J. Spath & W. Edmund Thirlkel Yale U.P., 1963. <;. 

[3]. Ibid.

[4]. Ibid.

[5]. McKeon. Aristotle. 236.

[6]. Ibid.

[7]. Aquinas. Commentary.

[8]. McKeon. Aristotle. 236-237.

[9]. Ibid. 237.

[10]. Aquinas. Commentary.

[11]. McKeon. Aristotle. 237.

[12]. Ibid.

[13]. Ibid. 237-238.

[14]. Ibid. 238.

[15]. Ibid.

[16]. Ibid.

[17]. Aquinas. Commentary.

Header Image: Raphael [Public domain], via Wikimedia Commons



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