Book II of the Physics may be divided into two sections. One (chapters i and 2) deals primarily with the meaning of nature. The other (chapters 3 to 9) is a study of causes.
Actually, in the first two chapters Aristotle is again occupied with the problem of principles, the burden of Book I. But the line of inquiry shifts. What he now examines is not exactly the principles of mobile being, but the principle of motion itself. This is called nature, and is opposed to art. Art is the principle of changes which result in fabricated or "artificial" things, whereas the products of nature are "natural" things. All in all it seems what Aristotle does in these two chapters is to determine more precisely the subject matter of natural science, which is to say, of the philosophy of nature.
To be noted, moreover, is that in studying the world of nature Aristotle is foremost a biologist. Many of the ideas he sets forth, the concept of nature in particular, bear the stamp of his familiarity and preoccupation with the biological order of things. To forget this is to miss the real meaning of much of his natural philosophy.
In Aristotle's view the existence of natural beings, or natures, has not to be proved; it is self-evident. Animals and their parts, plants, the elementsthese are all natural beings. In physics (in the Aristotelian sense) nature, like motion, is simply a postulate, a given, which Aristotle defines as follows: "Nature is a principle and cause of motion and rest in the thing in which it inheres primarily and as an attribute that is essential and not accidental." 1
a) Nature, therefore, is first a principle of motion. In its original use "nature" presumably meant the motion itself, but in time it came to denote the principle of motion. As for saying it is also a principle of "rest," this is a necessary inclusion because in Aristotelian physics rest is the motionlessness of something that can be moved. Like motion it has, therefore, to be accounted for by a cause. So, to the ancients, the nature of an element is the reason why it inclines to a particular place. Earth, one of the elements, is heavy. Earth, or its nature, is the reason why a heavy body falls and why it comes to rest after reaching its natural place.
b) Nature, moreover, is said to be an intrinsic principle. This distinguishes it from art. A manufactured thing, a coat or bed, does not, as manufactured, have a proper activity proceeding from its art-instilled form. As Aristotle observes, if a wooden bed were planted and grew, it would not come up a bed, but wood. The proper principle of a work of art is in the mind of the artist. But his mind or conception is an extrinsic principle; is it not, in the present meaning, a physical or natural principle.
In one sense, no doubt, man-made things have their characteristic form; but this form does not possess an activity of its own. If man-made things manifest a natural leaning or tendency, this comes from the materials in them, which retain their original properties within the artificial form imposed on them by the artist. By contrast with the artist, nature is an intrinsic principle, which both originates and determines the specific character of the activities embodied in its works.
c) Nature, finally, is something that inheres essentially, and not accidentally as a supervening attribute. This eliminates what Aristotle calls accidental causality. Thus, a man who is a doctor might heal himself, and the principle of healing would be within him, but not as an essential attributeone can be a man without being a doctor. That he was healed by his own art is, in the Aristotelian sense, accidental to him, and not a dispensation of his nature.
Usually when Aristotle refers to nature, he means the nature of an individual being. Sometimes, however, he speaks of it as a cosmic principle of life and movement: Nature with a capital letter, as is the fashion in some philosophical circles. But even with a capital, nature by Aristotle never has the build of a veritable world soul.
Another point of note is that nature is not the sole principle or cause of activity in a thing. There are also extrinsic causes, as is most evident in the inanimate world. Inanimate beings are known for being moved by another. Not that living things cannot so be moved; but the fact is more obvious in the other.