Having identified substance with essence, Aristotle attacks the view that substances are universals. This attack becomes effectively an attack on Plato’s Theory of Forms, and Aristotle argues forcefully that universal Forms cannot exist prior to the individual instances of them or be properly defined and so cannot play any role in science, let alone a fundamental role. He also argues against the suggestion that substances can be genus categories, like “animal” or “plant.” Humans and horses, unlike animals, have the property of “thisness”: the words human and horse pick out a particular kind of thing, whereas nothing particular is picked out by animal. Genuses are thus not specific enough to qualify as substances.
Book Eta contains a number of loosely connected points elaborating Aristotle’s views on substance. Aristotle associates an object’s matter with its potentiality and its form with its actuality. That is, matter is potentially a certain kind of substance and becomes that substance in actuality when it takes on the form of that substance. By associating substance with form and actuality, Aristotle infers a further connection between substance and differentia: differentia are those qualities that distinguish one species in a genus from another. Book Eta also contains reflections on the nature of names, matter, number, and definition.
This section of Aristotle’s Metaphysics is one of the most difficult and controversial texts in the history of philosophy. The problems are many-layered. First, there is the difficulty of the subject matter, which is fundamental and very abstract. Second, there is the difficulty of translation, as Aristotle uses many technical coinages that read oddly in Greek and are difficult to render clearly in English. Third, there is the difficulty of consistency, as Aristotle does not seem to reach any settled conclusions and does not have a clear central thrust to his argument. This third difficulty is related to a fourth: it seems the various chapters were written at different times and may have been roughly patched together by a later editor. For all these reasons, commentators are divided not only over how to interpret these passages of the Metaphysics but also over what they mean.
Aristotle’s metaphysics has been called a “metaphysics of substance” because he takes the fundamental questions of being to be equivalent to asking what substance is. In his Categories (see the Organon page TK), Aristotle distinguishes ten fundamental categories of being: substance, quantity, quality, relation, location, time, position, possession, doing, and undergoing. One of the consequences of dividing being into these ten categories is that there is no overarching concept of “being” that applies to all things. Animals have a categorically different kind of being than colors, and though we can say that both exist, they have totally different kinds of existence. Since Aristotle has stated that the Metaphysics studies beings qua being, that is, that it looks at what it means to say something exists at all, this division of being into ten categories would seem to complicate matters a bit. However, Aristotle argues both here and in the Categories that substance is the most fundamental kind of being, so the study of being is at heart the study of substance. The tricky part, it turns out, is sorting out what sorts of things qualify as substances.
Defining substances as species is a brilliant compromise between particularity and generality. Problems arise when we try to identify substances with individual entities such as Joe or my pet rock Tony, but problems also arise when we try to identify substances with genus categories such as animal or mineral, or universals such as justice or beauty. The problem posed by particular entities is that they have nonessential properties. Let’s imagine our friend Joe, who is clean shaven. Joe can grow a beard and still be Joe, so the property of beardlessness that holds of Joe is accidental, not essential. If we were to give a definition of “Joe,” we would not mention that he has no beard. On the other hand, if we were to give a description of Joe, we would have to mention that he has no beard if we wanted people to be able to distinguish Joe from his cousin Adam, who looks just like Joe but has a beard. So our description of Joe would make mention of properties that are not a part of his essence. In other words, to distinguish Joe from other people, we have to presuppose the existence of things, like beards, that are not essential to Joe. If substances are the fundamental building blocks of reality, their existence should not presuppose the existence of anything else. Since we cannot distinguish Joe without presupposing the existence of beards, Joe cannot be a substance. More generally, individual entities cannot be substances because they have nonessential properties that presuppose the existence of other things beyond themselves.
The trouble with genus categories and universals is that they do not have the property that Aristotle calls thisness. By thisness, he means a thing’s ability to be distinguished from among others of its kind. Joe has thisness because we can distinguish Joe from among other people, and humans as a species have thisness because they can be distinguished from among other animals. However, animals themselves are a general category, and we tend not to need to distinguish animals from other categories of anything. We can appreciate the importance of thisness by understanding Aristotle’s conception of definition. A definition consists of a genus term and differentia. For example, humans belong to the genus animal and can be differentiated from other animals by virtue of being rational. Humans, then, have thisness because they have differentia: they can be distinguished from others of their genus. A genus, on the other hand, has no differentia and so cannot be distinguished as clearly from other things. If substances are fundamental, then we must come to know all other things through our knowledge of substance. Knowledge would be impossible if we were unable to distinguish between the objects of our knowledge, so the fundamental objects of knowledge must have differentia, or thisness. Therefore, genus categories and universals cannot be substances.