Aristotle begins book Gamma (IV) of the Metaphysics by saying that after all, “There is a kind of knowledge that contemplates what is insofar as it is, and what belongs to it in its own right” (ch. 1, Sachs tr., p. 53).
To understand what this really means, we need to consider book Gamma as a whole, also taking into account in advance Aristotle’s disambiguation of “being” and “is” in book Delta (V). (There is another enumeration of the meanings of being in book Epsilon (VI), but it is does not really add anything to the one in book Delta.)
Here in book Gamma, he adds the new element that “Being is meant in more than one way, but pointing toward one meaning and some one nature rather than ambiguously…. just as every healthful thing points toward health…. For some things are called beings because they are independent things, others because they are attributes of independent things, others because they are ways into thinghood, or destructions or deprivations or qualities of thinghood, or are productive or generative of independent things, or of things spoken of in relation to independent things, or negations of any of these or of thinghood, on account of which we say even nonbeing is nonbeing. So just as there is one kind of knowledge of healthful things, this is similarly the case with the other things as well” (ch. 2, pp. 53-54).
Classically, “said in many ways” means said homonymously, like “flies” in “Time flies like an arrow” and “Fruit flies like bananas”. The same sound and spelling are used for different meanings that have no discernible relation to one another.
The comparison to health gives a pretty clear indication of what he wants to say here. But the “one thing” that is pointed to in the same way that health is pointed to is not “being”, but rather the concept of what Sachs calls independent things (“substances”).
In the disambiguation in book Delta, Aristotle says “just as many things are said to be in their own right as are meant by the modes of predication”. This is an allusion to the various ways in which things are said, which are enumerated and discussed in an elementary way in the Categories. Ousia (“substance”, or what Sachs calls “independent thinghood”) has a somewhat privileged place in this enumeration — we might say, just because it is the one that the others “point” to. Ousia will later turn out to be the “what it is” or “what it was to have been” of a thing.
The uses of “is” that he mentions are exclusively the transitive ones. He does not even mention any case like “Socrates is, full stop”. There is absolutely no mention of an “is” of existence, as opposed to the “is” of saying something about something. Saying something about something is the only role of “is” here, and this is strongly borne out by the remainder of the text of book Gamma.
Saying the what-it-is of something of that thing is for Aristotle the central, focal way of saying something about something in general. The other ways of saying something about something form what linguists might call a family of metonymies clustered around the saying of what-it-is. Metonymy involves indirectly referring to a thing by referring to one of its attributes. By contrast, Aristotle takes the what-it-is to refer directly to the thing (which is probably why Sachs calls the what-it-is the “thinghood” of the thing).
All the ways of saying something about something have in common that they are ways of saying something about something. In Sachs’ language, they are “modes of predication”. But the saying of what-it-is serves as a kind of paradigm for the rest.
The main body of book Gamma is actually a long polemic against the Sophists and the friends of Cratylus. It is about the conditions of rational discourse.
A number of the Sophists outraged Aristotle by making flagrantly self-contradictory assertions, and claiming a right to do so. Very uncharacteristically, Aristotle seems to lose his cool over this. He goes on and on about it, beating the dead horse into the ground. He does so because for him this is a violation of fundamental ethics.
To deliberately assert something and its contrary, or to claim a right to do so, is not just to talk nonsense. Aristotle implies it is deeply immoral — the deepest possible violation of intellectual integrity and the integrity of thought. As he says, it is completely impossible to have dialogue with someone who insists on this, and dialogue is the foundation of reason. For Aristotle, such a person hardly even qualifies as human.
Kant and Hegel treat unity of apperception not as something that spontaneously happens, but as the fulfillment of an ethical norm. When we commit ourselves to something by asserting it, we are then also morally committed to the assertion of what follows from it, and even more so to the denial of what is contrary to it. Aristotle’s outrage shows how strongly he shares this point of view.
The friends of Cratylus were radical Heracliteans. Heraclitus famously said that all things flow, and you cannot step in the same river twice. Cratylus claimed it would be more correct to say you cannot step in the same river once. Effectively, this means there is no such thing as being the same at all, so “same” has no real meaning. But if there is no sameness, there can be no contradiction, because contradiction is saying contrary things about the same thing. So the friends of Cratylus too ended up justifying what we would regard as self-contradictory statements.
So when Aristotle is concerned to assert that there is after all a knowledge of being as such and that it is the business of the philosopher to have it, all the evidence in book IV leads to the conclusion that what he is really saying is that the philosopher doesn’t just believe but knows that contraries are not true of the same thing in the same respect at the same time. This is simultaneously a genuine knowledge and a condition of any possible dialogue, a moral imperative as Kant would say.
The only other thing besides the principle of non-contradiction that Aristotle mentions in book IV as applying to being as such is mathematical axioms, which are similarly supposed to be true of all things whatsoever (he does not enumerate them).
Famously, Aquinas read Aristotle’s notion of “pointing toward one meaning” as a kind of analogy. The notion of an analogy of being is better considered as original to Aquinas. Analogy is a symmetrical relationship. If A is analogous to B, we should be able to conclude that B is similarly analogous to A.
But Aristotle’s example of the relation of health to healthful things is not symmetrical; health has a kind of logical primacy over healthful things, and Aristotle very explicitly gives the saying of what-it-is a similar primacy over its metonymic relatives corresponding to the other categories. That is the very means by which he gets from the non-univocity of being to a single concept. That concept is ousia — i.e., “substance”, “essence”, what Sachs calls “thinghood”, or the what-it-is of a thing.
Some commentators have argued that the subject of the Metaphysics is “ousiology”. That seems more accurate than the conventional “ontology”. In any case, Aristotle’s focus is on the conditions of meaningful saying, and especially on saying of what-it-is — not at all on being in the sense of existence.
In the big picture, existence as such is just not an important philosophical concept for Aristotle. What something is and why it is that way are what he is concerned with.
There will be a further level beyond this, in which we will further distinguish the saying of what something is by looking at it in potential and in act, where these terms are understood in a way that is independent of motion, and instead is oriented toward that-for-the-sake-of-which and the good.