Referring back to his logical work in the Categories, Aristotle opens book Zeta by asserting that substance is the primary category of being. Instead of considering what being is, we can consider what substance is.

Aristotle first rejects the idea that substance is the ultimate substrate of a thing, that which remains when all its accidental properties are stripped away. For example, a dog is more fundamental than the color brown or the property of hairiness that are associated with it. However, if we strip away all the properties that a dog possesses, we wind up with a substrate with no properties of its own. Since this substrate has no properties, we can say nothing about it, so this substrate cannot be substance.

Instead, Aristotle suggests that we consider substance as essence and concludes that substances are species. The essence of a thing is that which makes it that thing. For example, being rational is an essential property of being human, because a human without rationality ceases to be human, but being musical is not an essential property of being human, because a human without musical skill is still human. Individual people, or dogs, or tables, contain a mixture of essential and inessential properties. Species, on the other hand—for instance, people in general, dogs in general, or tables in general—contain only essential properties.

A substance can be given a definition that does not presuppose the existence of anything else. A snub, for example, is not a substance, because we would define a snub as “a concave nose,” so our definition of snub presupposes the existence of noses. A proper definition of a thing will list only its essential properties, and Aristotle asserts that only substances have essential properties or definitions. A snub nose, by contrast, has only accidental properties—properties like redness or largeness that may hold of some snubs but not of all—and per se properties—properties like concavity, which necessarily holds of all snubs but which is not essential.

Physical objects are composites of form and matter, and Aristotle identifies substance with form. The matter of an object is the stuff that makes it up, whereas the form is the shape that stuff takes. For example, the matter in a bronze sphere is the bronze itself, and the form is the spherical shape. Aristotle argues that form is primary because form is what gives each thing its distinctive nature.

Aristotle has argued that the definitions of substances cannot presuppose the existence of anything else, which raises the question of how there can be a definition that does not presuppose the existence of anything else. Presumably, a definition divides a whole into its constituent parts—for example, a human is defined as a rational animal—which suggests that a substance must in some way presuppose the existence of its constituent parts. Aristotle distinguishes between those cases where the parts of an object or definition are prior to the whole and those cases where the whole is prior to the parts. For example, we cannot understand the parts of a circle without first understanding the concept of circle as a whole; on the other hand, we cannot understand the whole of a syllable before we understand the letters that constitute its parts. Aristotle argues that, in the case of substance, the whole is prior to the parts. He has earlier associated substance with form and suggests that we cannot make sense of matter before we can conceive of its form. To say a substance can be divided by its definition is like saying a physical object can be divided into form and matter: this conceptual distinction is possible, but form and matter constitute an indivisible whole, and neither can exist without the other. Similarly, the parts of a definition of a substance are conceptually distinct, but they can only exist when they are joined in a substance.

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.

Species are the best candidates for substancehood because they have no nonessential properties but they also have thisness. While Joe and Adam have particular quirks that are true of only them, the species of human in general has no particular quirks outside the definition that holds of all humans. The essential properties of being human, like rationality, do not presuppose the existence of anything else because rationality is a part of what it is to be human. These essential properties also make humans distinguishable from ducks or rocks or trees. Hence, Aristotle reaches the remarkable conclusion that humans and ducks and rocks and trees are the most fundamental building blocks of reality. We can imagine particular humans or ducks or rocks or trees only because these species exist, and we can only infer general categories such as animal or mineral or plant from the existence of species that fit within these categories.