1069aOur inquiry is concerned with substance; for it is the principles and causes of substances that we are investigating. Indeed if the universe is to be regarded as a whole, substance is its first part; and if it is to be regarded as a succession,¹ even so substance is first, then quality, then quantity. Moreover, the latter hardly exist at all in the full sense, but are merely qualifications and affections of Being. Otherwise “not-white” and “not-straight” would also exist; at any rate we say that they too “are,” e.g., “it is not white.” Further, none of the other categories is separately existent. Even the ancients in effect testify to this, for it was of substance that they sought the principles and elements and causes. Present-day thinkers² tend to regard universals as substance, because genera are universal, and they hold that these are more truly principles and substances because they approach the question theoretically; but the ancients identified substance with particular things, e.g. fire and earth, and not with body in general. Now there are three kinds of substance. One is sensible (and may be either eternal³ or perishable; the latter, e.g. plants and animals, is universally recognized); of this we must apprehend the elements, whether they are one or many. Another is immutable , which certain thinkers hold to exist separately; some dividing it into two classes, others combining the Forms and the objects of mathematics into a single class, and others recognizing only the objects of mathematics as of this nature.⁴ The first two kinds of substance come within the scope of physics, since they involve motion; 1069bthe last belongs to some other science, if there is no principle common to all three. Sensible substance is liable to change. Now if change proceeds from opposites or intermediates—not however from all opposites (for speech is not white), but only from the contrary⁵—then there must be something underlying which changes into the opposite contrary; for the contraries⁶ do not change.

Further, something persists, whereas the contrary does not persist. Therefore besides the contraries there is some third thing, the matter . Now if change is of four kinds, in respect either of substance or of quality or of quantity or of place, and if change of substance is generation or destruction in the simple sense, and change of quantity is increase or decrease, and change of affection is alteration, and change of place is locomotion, then changes must be in each case into the corresponding contrary state. It must be the matter, then, which admits of both contraries, that changes. And since “that which is” is twofold, everything changes from that which is potentially to that which is actually; e.g. from potentially white to actually white. The same applies to increase and decrease. Hence not only may there be generation accidentally from that which is not, but also everything is generated from that which is, but is potentially and is not actually. And this is the “one” of Anaxagoras; for his “all things were together,”⁷ and the “mixture” of Empedocles and Anaximander and the doctrine of Democritus would be better expressed as “all things were together potentially, but not actually.” ⁸ Hence these thinkers must have had some conception of matter. All things which change have matter, but different things have different kinds; and of eternal things such as are not generable but are movable by locomotion have matter; matter, however, which admits not of generation, but of motion from one place to another.⁹ One might raise the question from what sort of “not-being” generation takes place; for not-being has three senses.¹⁰ If a thing exists through a potentiality, nevertheless it is not through a potentiality for any chance thing; different things are derived from different things. Nor is it satisfactory to say that “all things were together,” for they differ in their matter, since otherwise why did they become an infinity and not one? For Mind is one; so that if matter is also one, only that could have come to be in actuality whose matter existed potentially. The causes and principles, then, are three; two being the pair of contraries, of which one is the formula or form and the other the privation, and the third being the matter.¹¹

We must next observe¹² that neither matter nor form (I mean in the proximate sense) is generated. All change is of some subject by some agent into some object. 1070aThe agent is the immediate mover; the subject is the matter; and the object is the form. Thus the process will go on to infinity if not only the bronze comes to be round, but also roundness or bronze comes to be; there must, then, be some stopping-point. We must next observe that every substance is generated from something which has the same name (“substances” including not only natural but all other products). Things are generated either by art or by nature or by chance or spontaneously. Art is a generative principle in something else; nature is a generative principle in the subject itself¹³(for man begets man); the other causes are privations of these.¹⁴ There are three kinds of substance: (1.) matter, which exists individually in virtue of being apparent¹⁵(for everything which is characterized by contact and so not by coalescence is matter and substrate; e.g. fire, flesh and head; these are all matter, and the last is the matter of a substance in the strictest sense); (2.) the “nature”¹⁶(existing individually)—i.e. a kind of positive state which is the terminus of motion; and (3.) the particular combination of these, e.g. Socrates or Callias. In some cases the individuality does not exist apart from the composite substance (e.g., the form of a house does not exist separately, except as the art of building; nor are these forms liable to generation and destruction; there is a distinct sense in which “house” and “health” and every artificial product, considered in the abstract, do or do not exist¹⁷); if it does so at all, it does so in the case of natural objects. Hence Plato was not far wrong in saying¹⁸ that there are as many Forms as there are kinds of natural objects; that is if there are Forms distinct from the things of our world. Moving causes are causes in the sense of pre-existent things, but formal causes coexist with their effects. For it is when the man becomes healthy that health exists, and the shape of the bronze sphere comes into being simultaneously with the bronze sphere. Whether any form remains also afterwards is another question. In some cases there is nothing to prevent this, e.g. the soul may be of this nature (not all of it, but the intelligent part; for presumably all of it cannot be). Clearly then there is no need on these grounds for the Ideas to exist; for man begets man, the individual begetting the particular person. And the same is true of the arts, for the art of medicine is the formula of health.

In one sense the causes and principles are different for different things; but in another, if one speaks generally and analogically, they are the same for all. For the question might be raised whether the principles and elements of substances and of relations are the same or different; and similarly with respect to each of the other categories. But it is absurd that they should be the same for all; for then relations and substance would have the same constituents. 1070bWhat then can their common constituent be? For there is nothing common to and yet distinct from substance and the other predicable categories, yet the element is prior to that of which it is an element. Moreover substance is not an element of relations, nor is any of the latter an element of substance. Further, how can all the categories have the same elements? For no element can be the same as that which is composed of elements; e.g., neither B nor A can be the same as BA (nor indeed can any of the “intelligibles,”¹⁹ e.g. Unity or Being, be an element; for these apply in every case, even to composite things); hence no element can be either substance or relation. But it must be one or the other. Therefore the categories have not all the same elements. The truth is that, as we say, in one sense all things have the same elements and in another they have not. E.g., the elements of sensible bodies are, let us say, (1) as form, the hot, and in another sense the cold, which is the corresponding privation; as matter, that which directly and of its own nature is potentially hot or cold. And not only these are substances, but so are (2) the compounds²⁰ of which they are principles, and (3) any unity which is generated from hot and cold, e.g. flesh or bone; for the product of hot and cold must be distinct from them. These things, then, have the same elements and principles, although specifically different things have specifically different elements; we cannot, however, say that all things have the same elements in this sense, but only by analogy: i.e., one might say that there are three principles, form, privation and matter. But each of these is different in respect of each class of things, e.g., in the case of color they are white, black, surface; or again there is light, darkness and air, of which day and night are composed. And since not only things which are inherent in an object are its causes, but also certain external things, e.g. the moving cause, clearly “principle” and “element” are not the same; but both are causes. Principles are divided into these two kinds, and that which moves a thing or brings it to rest is a kind of principle and substance. Thus analogically there are three elements and four causes or principles; but they are different in different cases, and the proximate moving cause is different in different cases. Health, disease, body; and the moving cause is the art of medicine. Form, a particular kind of disorder, bricks; and the moving cause is the art of building. And since in the sphere of natural objects the moving cause of man is man, while in the sphere of objects of thought the moving cause is the form or its contrary, in one sense there are three causes and in another four. For in a sense the art of medicine is health, and the art of building is the form of a house, and man begets man; but besides these there is that which as first of all things moves all things.²¹