It might seem that the answer to this question is obvious: of course there is a highest kind — being. After all, someone might argue, everything exists. So the class that contains all and only beings must be the class with the greatest possible extension. In the Metaphysics, however, Aristotle argues that being is not a genus (998b23, 1059b31). According to Aristotle, every genus must be differentiated by some differentia that falls outside that genus. Hence, if being were a genus, it would have to be differentiated by a differentia that fell outside of it. In other words, being would have to be differentiated by some non-being, which, according to Aristotle, is a metaphysical absurdity. Although he does not explicitly make this claim, Aristotle’s argument, if cogent, would generalize to any proposal for a single highest kind. Hence, he does not think that there is one single highest kind. Instead, he thinks that there are ten: (1) substance; (2) quantity; (3) quality; (4) relatives; (5) somewhere; (6) sometime; (7) being in a position; (8) having; (9) acting; and (10) being acted upon (1b25–2a4). I shall discuss the first four of these kinds in detail in a moment. But doing so will take us into matters that, while interesting, nonetheless distract from the general nature of the scheme. So I will first discuss some of the general structures inherent in Aristotle’s second system of classification, and then proceed to a more detailed discussion.
In addition to positing ten highest kinds, Aristotle also has views about the structure of such kinds. Each kind is differentiated into species by some set of differentiae. In fact, the essence of any species, according to Aristotle, consists in its genus and the differentia that together with that genus defines the species. (It is for this reason that the highest kinds are, strictly speaking, indefinable — because there is no genus above a highest kind, one cannot define it in terms of its genus and a differentia.) Some of the species in various categories are also genera — they are, in other words, differentiated into further species. But at some point, there is a lowest species that is not further differentiated. Under these species, we can suppose, fall the particulars that belong to that species.
Now, if we accept the characterization of said-of and present-in that I have given, we can see that Aristotle’s two classificatory systems can, so to speak, be laid on top of each other. The resulting structure would look something like the following.
Substance Quantity Relatives Quality … Said-of
Not Present-InSaid-of
Present-InNot Said-of
Not Present-InNot Said-of
Present-In
Some features of this system are worth pointing out. First, as I have already noted, Aristotle gives pride of place in this scheme to primary substances. He says that were primary substances not to exist then no other entity would exist (2b6). As a result, Aristotle’s categorialism is firmly anti-Platonic. Whereas Plato treated the abstract as more real than material particulars, in the Categories Aristotle takes material particulars as ontological bedrock — to the extent that being a primary substance makes something more real than anything else, entities such as Socrates and a horse are the most real entities in Aristotle’s worldview. Moreover, among secondary substances, those at a lower level of generality are what Aristotle calls ‘prior in substance’ than those at a higher level (2b7). So, for instance, human is prior in substance than body. Whether this is to be interpreted in terms of the greater reality of the kind human is an open question. Nonetheless, Aristotle’s equating an increase in generality with a decrease in substantiality is at least in spirit strongly anti-Platonic.
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