2. The Categories
To understand the problems and project of Aristotle's Metaphysics, it is best to begin with one of his earlier works, the Categories. Although placed by long tradition among his logical works (see the discussion in the entry on Aristotle's logic), due to its analysis of the terms that make up the propositions out of which deductive inferences are constructed, the Categories begins with a strikingly general and exhaustive account of the things there are (ta onta)—beings. According to this account, beings can be divided into ten distinct categories. (Although Aristotle never says so, it is tempting to suppose that these categories are mutually exclusive and jointly exhaustive of the things there are.) They include substance, quality, quantity, and relation, among others. Of these categories of beings, it is the first, substance (ousia), to which Aristotle gives a privileged position.
Substances are unique in being independent things; the items in the other categories all depend somehow on substances. That is, qualities are the qualities of substances; quantities are the amounts and sizes that substances come in; relations are the way substances stand to one another. These various non-substances all owe their existence to substances—each of them, as Aristotle puts it, exists only ‘in’ a subject. That is, each non-substance “is in something, not as a part, and cannot exist separately from what it is in” (Cat. 1a25). Indeed, it becomes clear that substances are the subjects that these ontologically dependent non-substances are ‘in’.
Each member of a non-substance category thus stands in this inherence relation (as it is frequently called) to some substance or other—color is always found in bodies, knowledge in the soul. Neither whiteness nor a piece of grammatical knowledge, for example, is capable of existing on its own. Each requires for its existence that there be some substance in which it inheres.
In addition to this fundamental inherence relation across categories, Aristotle also points out another fundamental relation that obtains between items within a single category. He describes this as the relation of “being said of a subject,” and his examples make clear that it is the relation of a more general to a less general thing within a single category. Thus, man is ‘said of’ a particular man, and animal is ‘said of’ man, and therefore, as Aristotle points out, animal is ‘said of’ the particular man also. The ‘said of’ relation, that is to say, is transitive (cf. 1b10). So the genus (e.g., animal) is ‘said of’ the species (e.g., man) and both genus and species are ‘said of’ the particular. The same holds in non-substance categories. In the category of quality, for example, the genus (color) is ‘said of’ the species (white) and both genus and species are ‘said of’ the particular white. There has been considerable scholarly dispute about these particulars in nonsubstance categories. For more detail, see the supplementary document:
Nonsubstantial Particulars
The language of this contrast (‘in’ a subject vs. ‘said of’ a subject) is peculiar to the Categories, but the idea seems to recur in other works as the distinction between accidental vs. essential predication. Similarly, in works other than the Categories, Aristotle uses the label ‘universals’ (ta katholou) for the things that are “said of many;” things that are not universal he calls ‘particulars’ (ta kath’ hekasta). Although he does not use these labels in the Categories, it is not misleading to say that the doctrine of the Categories is that each category contains a hierarchy of universals and particulars, with each universal being ‘said of’ the lower-level universals and particulars that fall beneath it. Each category thus has the structure of an upside-down tree.[2] At the top (or trunk) of the tree are the most generic items in that category[3] (e.g., in the case of the category of substance, the genus plant and the genus animal); branching below them are universals at the next highest level, and branching below these are found lower levels of universals, and so on, down to the lowest level universals (e.g., such infimae species as man and horse); at the lowest level—the leaves of the tree—are found the individual substances, e.g., this man, that horse, etc.
The individuals in the category of substance play a special role in this scheme. Aristotle calls them “primary substances” (prôtai ousiai) for without them, as he says, nothing else would exist. Indeed, Aristotle offers an argument (2a35–2b7) to establish the primary substances as the fundamental entities in this ontology. Everything that is not a primary substance, he points out, stands in one of the two relations (inhering ‘in’, or being ‘said of’) to primary substances. A genus, such as animal, is ‘said of’ the species below it and, since they are ‘said of’ primary substances, so is the genus (recall the transitivity of the ‘said of’ relation). Thus, everything in the category of substance that is not itself a primary substance is, ultimately, ‘said of’ primary substances. And if there were no primary substances, there would be no “secondary” substances (species and genera), either. For these secondary substances are just the ways in which the primary substances are fundamentally classified within the category of substance. As for the members of non-substance categories, they too depend for their existence on primary substances. A universal in a non-substance category, e.g., color, in the category of quality, is ‘in’ body, Aristotle tells us, and therefore in individual bodies. For color could not be ‘in’ body, in general, unless it were ‘in’ at least some particular bodies. Similarly, particulars in non-substance categories (although there is not general agreement among scholars about what such particulars might be) cannot exist on their own. E.g., a determinate shade of color, or a particular and non-shareable bit of that shade, is not capable of existing on its own—if it were not ‘in’ at least some primary substance, it would not exist. So primary substances are the basic entities—the basic “things that there are”—in the world of the Categories.