The unity of the many senses of being is maintained, for Aristotle, by their common reference to the primary being of substance. This he explains in Book Γ, chapter 2, of Metaphysics:
The term being is used in many senses, but with reference to one thing and to some one nature and not equivocally. Thus everything healthy is related to health, one thing because it preserves health, another because it causes it, another because it is a sign of it (as urine) and still another because it is receptive of it. The term medical is related in a similar way to the art of medicine; for one thing is called medical because it possesses the art of medicine, another because it is receptive of it, and still another because it is the act of those who have the art of medicine. We can take other words which are used in a way similar to these. And similarly there are many senses in which the term being is used, but each is referred to a first principle. For some things are called beings because they are substances; others because they are affections of substances; others because they are a process toward substance, or corruptions or privations or qualities of substance, or because they are productive or generative principles of substance, or of things which are related to substance, or the negation of some of these or of substance. For this reason too we say that non-being is non-being. (1003a34-1003b11)
This is essentially a matter of definition. To say that being is meant in many senses is to assert that it has many definitions - or rather, that it has no definition, because it does not have the kind of unity of a genus with is differentiated into many species. Thus, all the "kinds" of being - the ten categories - are not species of being, having in common a single, generic, definition. At first sight, then, it would seem to be the case that "being" is simply equivocal. But this Aristotle denies. Being is not simply equivocal - rather, it is equivocal in a qualified sense, because it is said of all the categories always with reference to the single and primary category of substance. Substance is being most properly speaking; and all of the accidents - as well generative or corruptive processes, or non-being itself - are named only with a focal reference to the being of substance.
This is primarily a logical claim, and yet it must certainly have something to do with metaphysics, since it is in the Metaphysics that Aristotle employs this principle. It is important and highly interesting to note that, although logic and metaphysics are distinct sciences, they alone of all the sciences are the only two that appear to be equal in scope: logic and metaphysics both study the entirety of being; the difference is in the ratio or formality under which being is considered, either as thought or simply as being. This is indeed why logic is of the utmost importance in the practice of all the sciences: because it is truly universal, in a sense - not in the architectonic, governing sense in which metaphysics is universal. Logic is, so to speak, universally useful (it is a practical science, after all - one of the seven liberal arts). This is seen, for example, in Aristotle's discussion of the good in the Nicomachean Ethics, where "good" is not said univocally, but with a focal reference to some final good which constitutes human happiness. To this claim about how we speak of the good, there also corresponds a profound metaphysics of the good. Thus, although a logical claim is not per se a metaphysical claim, one might have good reason to think that to every logical claim about things there corresponds a metaphysical or ontological truth about the structure of reality.
Thus, in this particular case, the logical observation is that, in the way we speak, it is the mode of signifying something as substance which first and foremost bears the notion of the subject in any predication. We can predicate things of any of the other nine categories too - but we never predicate substance of an accident, and we always end up predicating the accidents of a substance. Substance is the root of all predication, the final and fundamental condition for speaking predicates. Now, it is not necessarily true that from every logical claim, the corresponding metaphysical claim can be directly inferred; indeed, it is quite dangerous to attempt to do so. (To one who tries to follow this method, Aristotle's claim about the priority of form in book Z would seem unintelligible, as we have seen in my previous post. Hopefully I will get to write in more detail later on the relation between logic and metaphysics.) Nonetheless, by proper method in the science of metaphysics on its own terms, one can discover the ontological basis for all of logic itself. And on the basis of such method, one can see, with Aristotle, that it is indeed substance which communicates being to all other modes of being. For substance alone is that mode of being which is the most independent, self-subsistent, and determinate; all other modes of being are contingent upon the being of substance itself. Logical analogy seems to correspond to a real structure within the fabric of reality.
In my next post, I will introduce an important Neoplatonic philosopher to this blog: Syrianus, master of the academy in Athens from about 431-437 AD, and commentator on parts of Aristotle's Metaphysics. Syrianus is perhaps best known for being the teacher of Proclus, who himself became known as one of the greatest and most influential philosophers of Late Antiquity. Syrianus and Proclus were both thoroughly acquainted with the works of both Plato and Aristotle, and they appeared within the context of a trend in Neoplatonism towards the harmonization of these two great masters. However, they were far from uncritical of Aristotle, and they were well aware of Aristotle's own heavy-handed criticisms of the Platonic theory of Forms. I hope, next week, to take a look at Syrianus' use of Aristotle's own notion of πρὀς ἕν predication - the doctrine of analogy or focal meaning - to refute Aristotle's objections against the Forms.