It is obvious that even of those things which are thought to be substances the majority are potentialities; both the parts of living things (for none of them has a separate substantial existence; and when they are separated, although they still exist, they exist as matter), and earth, fire and air; for none of these is one thing —they are a mere aggregate before they are digested and some one thing is generated from them. It might be supposed very reasonably that the parts of living things and the corresponding parts of their vital principle are both, i.e. exist both actually and potentially, because they contain principles of motion derived from something in their joints; and hence some animals¹ live even when they are divided. Nevertheless it is only potentially that all of them will exist when they are one and continuous by nature and not by force or concretion; for this sort of thing is malformation.² And since “unity” has the same variety of senses as “being,” and the substance of Unity is one, and things whose substance is numerically one are numerically one, evidently neither Unity nor Being can be the substance of things, just as neither “being an element” or “principle” can be the substance; but we ask what the principle is so that we may refer to something more intelligible.³ Now of these concepts Being and Unity are more nearly substance than are principle, element and cause; but not even the former are quite substance, since nothing else that is common is substance; for substance belongs to nothing except itself and that which contains it and of which it is the substance. Again, Unity cannot exist in many places at the same time, but that which is common is present in many things at the same time. Hence it is clear that no universal exists in separation apart from its particulars. The exponents of the Forms are partly right in their account when they make the Forms separate; that is, if the Forms are substances, but they are also partly wrong, since by “Form” they mean the “one-over-many.” ⁴ The reason for this is that they cannot explain what are the imperishable substances of this kind which exist besides particular sensible substances; so they make them the same in kind as perishable things (for these we know); i.e., they make “Ideal Man” and “Ideal Horse,” adding the word “Ideal” to the names of sensible things. However, I presume that even if we had never seen the stars, 1041anone the less there would be eternal substances besides those which we knew; and so in the present case even if we cannot apprehend what they are, still there must be eternal substances of some kind. It is clear, then, both that no universal term is substance and that no substance is composed of substances.

As for what and what sort of thing we mean by substance, let us explain this by making, as it were, another fresh start. Perhaps in this way we shall also obtain some light upon that kind of substance which exists in separation from sensible substances. Since, then, substance is a kind of principle and cause, we had better pursue our inquiry from this point. Now when we ask why a thing is, it is always in the sense “why does A belong to B?” To ask why the cultured man is a cultured man is to ask either, as we have said, why the man is cultured, or something else. Now to ask why a thing is itself is no question; because when we ask the reason of a thing the fact must first be evident; e.g., that the moon suffers eclipse; and “because it is itself” is the one explanation and reason which applies to all questions such as “why is man man?” or “why is the cultured person cultured?” (unless one were to say that each thing is indivisible from itself, and that this is what “being one” really means); but this, besides being a general answer, is a summary one.⁵ We may, however, ask why a man is an animal of such-and-such a kind. It is clear, then, that we are not asking why he who is a man is a man; therefore we are asking why A, which is predicated of B, belongs to B. (The fact that A does belong to B must be evident, for if this is not so, the question is pointless.) E.g., “Why does it thunder?” means “why is a noise produced in the clouds?” for the true form of the question is one thing predicated in this way of another. Or again, “why are these things, e.g. bricks and stones, a house?” Clearly then we are inquiring for the cause (i.e., to speak abstractly, the essence); which is in the case of some things, e.g. house or bed, the end , and in others the prime mover—for this also is a cause. We look for the latter kind of cause in the case of generation and destruction, but for the former also in the case of existence. What we are now looking for is most obscure when one term is not predicated of another; 1041be.g. when we inquire what man is; because the expression is a simple one not analyzed into subject and attributes. We must make the question articulate before we ask it; otherwise we get something which shares the nature of a pointless and of a definite question. Now since we must know that the fact actually exists, it is surely clear that the question is “why is the matter so-and-so?” e.g. “why are these materials a house?” Because the essence of house is present in them. And this matter, or the body containing this particular form, is man. Thus what we are seeking is the cause (i.e. the form) in virtue of which the matter is a definite thing; and this is the substance of the thing. Clearly then in the case of simple entities⁶ inquiry and explanation are impossible; in such cases there is a different mode of inquiry. Now since that which is composed of something in such a way that the whole is a unity; not as an aggregate is a unity, but as a syllable is⁷—the syllable is not the letters, nor is BA the same as B and A; nor is flesh fire and earth; because after dissolution the compounds, e.g. flesh or the syllable, no longer exist; but the letters exist, and so do fire and earth. Therefore the syllable is some particular thing; not merely the letters, vowel and consonant, but something else besides. And flesh is not merely fire and earth, or hot and cold, but something else besides. Since then this something else must be either an element or composed of elements, (a) if it is an element, the same argument applies again; for flesh will be composed of this and fire and earth, and again of another element, so that there will be an infinite regression. And (b) if it is composed of elements, clearly it is composed not of one (otherwise it will itself be that element) but of several; so that we shall use the same argument in this case as about the flesh or the syllable. It would seem, however, that this “something else” is something that is not an element, but is the cause that this matter is flesh and that matter a syllable, and similarly in other cases. And this is the substance of each thing, for it is the primary cause of its existence. And since, although some things are not substances, all substances are constituted in accordance with and by nature, substance would seem to be this “nature,” which is not an element but a principle.⁸ An element is that which is present as matter in a thing, and into which the thing is divided; e.g., A and B are the elements of the syllable.