Aristotle himself described his subject matter in a variety of ways: as ‘first philosophy’, or ‘the study of being qua being’, or ‘wisdom’, or ‘theology’. A comment on these descriptions will help to clarify Aristotle's topic.
In Metaphysics A.1, Aristotle says that “all men suppose what is called wisdom (sophia) to deal with the first causes (aitia) and the principles (archai) of things” (981b28), and it is these causes and principles that he proposes to study in this work. It is his customary practice to begin an inquiry by reviewing the opinions previously held by others, and that is what he does here, as Book A continues with a history of the thought of his predecessors about causes and principles.
These causes and principles are clearly the subject matter of what he calls ‘first philosophy’. But this does not mean the branch of philosophy that should be studied first. Rather, it concerns issues that are in some sense the most fundamental or at the highest level of generality. Aristotle distinguished between things that are “better known to us” and things that are “better known in themselves,” and maintained that we should begin our study of a given topic with things better known to us and arrive ultimately at an understanding of things better known in themselves. The principles studied by ‘first philosophy’ may seem very general and abstract, but they are, according to Aristotle, better known in themselves, however remote they may seem from the world of ordinary experience. Still, since they are to be studied only by one who has already studied nature (which is the subject matter of the Physics), they are quite appropriately described as coming “after the Physics.”
Aristotle's description ‘the study of being qua being’ is frequently and easily misunderstood, for it seems to suggest that there is a single (albeit special) subject matter—being qua being—that is under investigation. But Aristotle's description does not involve two things—(1) a study and (2) a subject matter (being qua being)—for he did not think that there is any such subject matter as ‘being qua being’. Rather, his description involves three things: (1) a study, (2) a subject matter (being), and (3) a manner in which the subject matter is studied (qua being).
Aristotle's Greek word that has been Latinized as ‘qua’ means roughly ‘in so far as’ or ‘under the aspect’. A study of x qua y, then, is a study of x that concerns itself solely with the y aspect of x. So Aristotle's study does not concern some recondite subject matter known as ‘being qua being’. Rather it is a study of being, or better, of beings—of things that can be said to be—that studies them in a particular way: as beings, in so far as they are beings.
Of course, first philosophy is not the only field of inquiry to study beings. Natural science and mathematics also study beings, but in different ways, under different aspects. The natural scientist studies them as things that are subject to the laws of nature, as things that move and undergo change. That is, the natural scientist studies things qua movable (i.e., in so far as they are subject to change). The mathematician studies things qua countable and measurable. The metaphysician, on the other hand, studies them in a more general and abstract way—qua beings. So first philosophy studies the causes and principles of beings qua beings. In Γ.2, Aristotle adds that for this reason it studies the causes and principles of substances (ousiai). We will explain this connection in Section 3 below.
In Book E, Aristotle adds another description to the study of the causes and principles of beings qua beings. Whereas natural science studies objects that are material and subject to change, and mathematics studies objects that although not subject to change are nevertheless not separate from (i.e., independent of) matter, there is still room for a science that studies things (if indeed there are any) that are eternal, not subject to change, and independent of matter. Such a science, he says, is theology, and this is the “first” and “highest” science. Aristotle's identification of theology, so conceived, with the study of being qua being has proved challenging to his interpreters. We will deal with this issue in a future update, when Section 14 is completed.
Finally, we may note that in Book B, Aristotle delineates his subject matter in a different way, by listing the problems or perplexities (aporiai) he hopes to deal with. Characteristic of these perplexities, he says, is that they tie our thinking up in knots. They include the following, among others: Are sensible substances the only ones that exist, or are there others besides them? Is it kinds or individuals that are the elements and principles of things? And if it is kinds, which ones: the most generic or the most specific? Is there a cause apart from matter? Is there anything apart from material compounds? Are the principles limited, either in number or in kind? Are the principles of perishable things themselves perishable? Are the principles universal or particular, and do they exist potentially or actually? Are mathematical objects (numbers, lines, figures, points) substances? If they are, are they separate from or do they always belong to sensible things? And (“the hardest and most perplexing of all,” Aristotle says) are unity and being the substance of things, or are they attributes of some other subject? In the remainder of Book B, Aristotle presents arguments on both sides of each of these issues, and in subsequent books he takes up many of them again. But it is not always clear precisely how he resolves them, and it is possible that Aristotle did not think that the Metaphysics contains definitive solutions to all of these perplexities.