The title of the Metaphysics (τὰ μετὰ τὰ φυσικά) is not Aristotle's. He describes his inquiry as "theology" and "first philosophy."

(The Metaphysics, as it has come down to us, is divided into fourteen books named with the first thirteen letters of the Greek alphabet (I: Alpha (Α), II: little Alpha (α), III: Beta (Β), IV: Gamma (Γ), V: Delta (Δ), VI: Epsilon (Ε), VII: Zeta (Ζ), VIII: Eta (Η), IX: Theta (Θ), X: Iota (Ι), XI: Kappa (Κ), XII: Lambda (Λ), XIII: Mu (Μ), and XIV: Nu (Ν). The title Metaphysics was imposed by later editors. (In his catalogue of Aristotle's writings, Diogenes Laertius does not list a work entitled Metaphysics.) The title refers to the position of the Metaphysics within the corpus--it comes after the physical works. Books II, V, and XI seem to be later additions, and the underlying work does not reads less like a finished product and more like a series of notes written at different times.)

Theology is First Philosophy

Aristotle's description raises a question about whether his inquiry is into more than one subject. First philosophy is the study of "being as being." Theology is a study of divine objects. So theology and first philosophy would appear to to be different subjects. Theology is a study of some of the beings there are, the divine objects. First philosophy studies being in general.

What, if anything, according to Aristotle, unifies theology and first philosophy so that his inquiry in the Metaphysics is into a single subject?

The answer to this question is not completely clear, but Aristotle's idea seems to be that there are "ways of being." He seems to think that the way of being that characterizes the existence of divine objects is first among the ways. Divine objects are instances of being in the primary way. Because the being of divine objects is being without qualification, and because all other ways of being are qualifications of the way of divine objects, theology is first philosophy. Theology provides the explanation of primary being and thus being in general.

"The theoretical sciences (θεωρητικαὶ) are to be preferred to the other sciences, and theology to the other theoretical sciences, but one might wonder whether first philosophy is universal or deals with some genus and one kind (ἡ πρώτη φιλοσοφία καθόλου ἐστὶν ἢ περί τι γένος καὶ φύσιν τινὰ μίαν). .... If there is not some other substance (οὐσία) besides those which are naturally composed, physics will be the primary science (ἡ φυσικὴ ἂν εἴη πρώτη ἐπιστήμη); but if there is a substance which is immutable (ἀκίνητος), the science which studies this will be prior to physics, and will be first philosophy, and universal in this way, that it is primary. It will be for this science to study being as being (ὄντος ᾗ ὂν); what it is, and what the attributes are which belong to it as being" (Metaphysics VI.1.1026a).

What things are Substances

"Indeed long ago, now, and always, what is sought after and always puzzled over, what is being (ὄν), is the question what is substance (οὐσία). Some say that it is one; others, more than one; some, finite others, infinite. And so for us too our chief and primary and practically our only concern is to investigate what this way of being is" (Metaphysics VII.1.1028b).

Aristotle thinks that objects are beings. Humans, for example, are beings. They are human beings. If we ask what their being is, we are asking what their "substance" (οὐσία) is. For Aristotle, by the time of the Metaphysics, the answer being considered is that a form in matter is the substance of a human being. Aristotle wants to know what that is and how it is a substance.

(The noun οὐσία derives from the feminine present participle of εἰμί (“I am, I exist”). The infinitive form of εἰμί is εἶναι ("to be"). The singular nominative/vocative present participles of εἰμί are ὤν (masculine), οὖσα (feminine), ὄν (neuter).) The word οὐσία is often translated into Latin as substantia and into English as substance. This translation reflects the idea that things in other categories of existence depend for their being on the substances. The Latin substantia more literally translates the Greek word ὑπόστασις, which is a compound of ὑπό ("under") and ἵστημι ("I stand").

("Now how about such things as this, Simmias? Do we think there is such a thing as the just itself, or not? ... And the beautiful and the good? ... I am speaking of all such things, as size, health, strength, and in short ... the reality (οὐσίας) of all other things, that which each of them is (ὃ τυγχάνει ἕκαστον ὄν)" (Phaedo 65d). "If, as we are always saying, the beautiful exists, and the good, and every being (οὐσία) of that kind..." (Phaedo 76d). "Let us then return to those same things with which we were dealing earlier, to that reality (οὐσία) of whose existence we are giving an account in our questions and answers..." (Phaedo 78c). "...for our soul was said to exist also before it came into the body, just as the reality (οὐσία) does that is of the kind that we qualify by the words 'what it is'..." (Phaedo 92d). "And you would loudly exclaim that you do not know how else each thing can come to be except by sharing in the particular reality (οὐσίας) in which it shares..." (Phaedo 101c).)

The development of Aristotle's thought

Aristotle's understanding of being in terms of substance seems to change in the time between the Categories to the Metaphysics. His discussion on these works is difficult, but he seems to begin his thinking about substance in the Categories primarily as critic of Plato's ontology. It is only later, in the central books of the Metaphysics, that he sees something important in Platonism and its commitment to forms. Aristotle does not explicitly comment in the Metaphysics on his earlier views in the Categories, but he seems to conclude that

• he was wrong about what he identified as substance in the Categories
• the primary way of being is the way that belongs to forms
• divine objects have this way of being
• the way of being of natural bodies is a qualification of the primary way of being.

"We must consider what things are substances (οὐσίαι); and whether there are any substances besides the sensible substances (τὰς αἰσθητὰς), or not; and how sensible substances exist; and whether there is any separable substance (χωριστὴ οὐσία) (and if so, why and how) or no substance besides the sensible ones" (Metaphysics VII.2.1028b).

[I]t is accepted that separability and individuality belong especially to substance (τὸ χωριστὸν καὶ τὸ τόδε τι ὑπάρχειν δοκεῖ μάλιστα τῇ οὐσίᾳ). Hence it would seem that the form and the combination of form and matter (τὸ εἶδος καὶ τὸ ἐξ ἀμφοῖν) are more truly substance than matter is. The substance, then, which consists of both—I mean of matter and form—may be dismissed, since it is posterior and obvious. Matter too is in a sense evident. We must consider the third type, for this is the most perplexing (ἀπορωτάτη)" (Metaphysics VII.3.1029a).

Perseus Digital Library: Aristotle, Metaphysics
Henry George Liddell, Robert Scott, A Greek-English Lexicon:
οὐσία, ousia, noun, "substance,"
ὑπόστασις, hypostasis, noun, "standing under, supporting"

Charlton T. Lewis, Charles Short, A Latin Dictionary:
substantia, noun, "that of which a thing consists"

"Traditionally οὐσία has been rendered by 'substance.' The reason for this is that, on the view Aristotle puts forward in the Categories, properties depend for their being on objects in that objects are their ultimate subjects, they are what ultimately underlies everything else. Indeed, objects in the Categories are characterized by the very fact that they are the ultimate subjects which underlie everything, whereas there is nothing that underlies them as their subject. It is because of this characterization that the rendering 'substance' seems appropriate" (Michael Frede, "Substance in Aristotle's Metaphysics," 73. Essays in Ancient Philosophy, Michael Frede (University of Minnesota Press, 1987), 72-80).

"[L]et us try to understand how it is that theology is not concerned only with a particular kind of beings, but with a particular way of being, peculiar to its objects.... By distinguishing a kind of beings and a way of being I mean to make a distinction of the following sort. Horses are a kind of beings, and camels are a different kind of beings, but neither horses nor camels have a distinctive way of being, peculiar to them; they both have the way of being of natural substances, as opposed to, e.g., numbers which have the way of magnitudes, or qualities which have a yet different way of being. ... Though physics deals only with a particular part of reality, namely, natural substances, its objects do have a distinctive way of being, namely, the way of natural substances" (Michael Frede, "The Unity of General and Special Metaphysics: Aristotle's Conception of Metaphysics," 85-86. Essays in Ancient Philosophy, Michael Frede (University of Minnesota Press, 1987), 81-95).

"Let us call the history of the changes an object has undergone, the history of the object; we shall want the substance of an object to be such that with reference to it we can explain how, despite all the changes, it is the history of one object. We also think an object might have had a history quite different from the one it actually had yet have been the same object; this, too, is to be explained in terms of substance. Furthermore, the substance must be an individual, since we are looking for the real individuals in the category of substance which are to explain the individuality of ordinary individual objects. Finally, there must be some sort of asymmetry between substances and properties, on the basis of which we can say of properties and everything else that exists that they depend on substances for their existence, but that substances do not, in any way, depend on properties for their existence. These are the requirements Aristotle lays down in the Metaphysics, when he says a substance must be a subject (hypokeimenon), "a this" (tode ti), and an independently existing entity (choriston)" (Michael Frede, "Individuals in Aristotle, 64-65. Essays in Ancient Philosophy, Michael Frede (University of Minnesota Press, 1987), 49-71).

"According to Aristotle, the form satisfies these [three] requirements and thus is the substance" (Michael Frede, "Individuals in Aristotle, 65. Essays in Ancient Philosophy, Michael Frede (University of Minnesota Press, 1987), 49-71).

"[W]e have to wonder why Aristotle now is considering matter, form, and the composite of both as possible ultimate subjects of predication. For none of these is identical with the particular objects of the Categories. This goes without saying for matter and form. But it also seems to be true for the composite of matter and form. It is true that traditionally the composite has been identified with the concrete, particular object. But the concrete, particular object, as we are familiar with it, actually is a composite not just of matter and form, but also of a large number of accidents; it is an object of a certain size, weight, color, and the like, i.e., a complex of entities. Hence, one should not assume without further argument that the composite of matter and form is to be identified without qualification with the concrete particular" (Michael Frede, "Substance in Aristotle's Metaphysics," 74. Essays in Ancient Philosophy, Michael Frede (University of Minnesota Press, 1987), 72-80).