3. The Role of Substance in the Study of Being Qua Being
The Categories leads us to expect that the study of being in general (being qua being) will crucially involve the study of substance, and when we turn to the Metaphysics we are not disappointed. First, in Metaphysics Γ Aristotle argues in a new way for the ontological priority of substance; and then, in Books Ζ, Η, and Θ, he wrestles with the problem of what it is to be a substance. We will begin with Γ's account of the central place of substance in the study of being qua being.
As we noted above, metaphysics (or, first philosophy) is the science which studies being qua being. In this respect it is unlike the specialized or departmental sciences, which study only part of being (only some of the things that exist) or study beings only in a specialized way (e.g., only in so far as they are changeable, rather than in so far as they are beings).
But ‘being’, as Aristotle tells us in Γ.2, is “said in many ways”. That is, the verb ‘to be’ (einai) has different senses, as do its cognates ‘being’ (on) and ‘entities’ (onta). So the universal science of being qua being appears to founder on an equivocation: how can there be a single science of being when the very term ‘being’ is ambiguous?
Consider an analogy. There are dining tables, and there are tide tables. A dining table is a table in the sense of a smooth flat slab fixed on legs; a tide table is a table in the sense of a systematic arrangement of data in rows and columns. But there is not a single sense of ‘table’ which applies to both the piece of furniture at which I am writing these words and to the small booklet that lies upon it. Hence it would be foolish to expect that there is a single science of tables, in general, that would include among its objects both dining tables and tide tables. Tables, that is to say, do not constitute a single kind with a single definition, so no single science, or field of knowledge, can encompass precisely those things that are correctly called ‘tables’.
If the term ‘being’ were ambiguous in the way that ‘table’ is, Aristotle's science of being qua being would be as impossible as a science of tables qua tables. But, Aristotle argues in Γ.2, ‘being’ is not ambiguous in this way. ‘Being’, he tells us, is ‘said in many ways’ but it is not merely (what he calls) ‘homonymous’, i.e., sheerly ambiguous. Rather, the various senses of ‘being’ have what he calls a ‘pros hen’ ambiguity—they are all related to a single central sense. (The Greek phrase ‘pros hen’ means “in relation to one.”)
Aristotle explains his point by means of some examples that he takes to be analogous to ‘being’. Consider the terms ‘healthy’ and ‘medical’. Neither of these has a single definition that applies uniformly to all cases: not every healthy (or medical) thing is healthy (medical) in the same sense of ‘healthy’ (‘medical’). There is a range of things that can be called ‘healthy’: people, diets, exercise, complexions, etc. Not all of these are healthy in the same sense. Exercise is healthy in the sense of being productive of health; a clear complexion is healthy in the sense of being symptomatic of health; a person is healthy in the sense of having good health.
But notice that these various senses have something in common: a reference to one central thing, health, which is actually possessed by only some of the things that are spoken of as ‘healthy’, namely, healthy organisms, and these are said to be healthy in the primary sense of the term. Other things are considered healthy only in so far as they are appropriately related to things that are healthy in this primary sense.
The situation is the same, Aristotle claims, with the term ‘being’. It, too, has a primary sense as well as related senses in which it applies to other things because they are appropriately related to things that are called ‘beings’ in the primary sense. The beings in the primary sense are substances; the beings in other senses are the qualities, quantities, etc., that belong to substances. An animal, e.g., a horse, is a being, and so is a color, e.g, white, a being. But a horse is a being in the primary sense—it is a substance—whereas the color white (a quality) is a being only because it qualifies some substance. An account of the being of anything that is, therefore, will ultimately have to make some reference to substance. Hence, the science of being qua being will involve an account of the central case of beings—substances.