Philosophy is also concerned with logic and the principles of demonstration, which are supremely general, and hence concerned with being itself. The most fundamental principle is the principle of noncontradiction: nothing can both be something and not be that same something. Aristotle defends this principle by arguing that it is impossible to contradict it coherently. Connected to the principle of non-contradiction is the principle of the excluded middle, which states that there is no middle position between two contradictory positions. That is, a thing is either x or not-x, and there is no third possibility. Book Gamma concludes with an attack on several general claims of earlier philosophers: that everything is true, that everything is false, that everything is at rest, and that everything is in motion.
Book Delta consists of the definitions of about forty terms, some of which feature prominently in the rest of the Metaphysics, such as principle, cause, nature, being, and substance. The definitions specify precisely how Aristotle uses these terms and often distinguish between different uses or categories of the terms.
Book Epsilon opens by distinguishing philosophy from the sciences not just on the basis of its generality but also because philosophy, unlike the sciences, takes itself as a subject of inquiry. The sciences can be divided into practical, productive, and theoretical. The theoretical sciences can be divided further into physics, mathematics, and theology, or first philosophy, which studies first principles and causes.
We can look at being in four different ways: accidental being, being as truth, the category of being, and being in actuality and potentiality. Aristotle considers the first two in book Epsilon and examines the category of being, or substance, in books Zeta and Eta, and being in actuality and potentiality in book Theta. Accidental being covers the kinds of properties that are not essential to a thing described. For example, if a man is musical, his musicality is accidental since being musical does not define him as a man and he would still be a man even if he were not musical. Accidental being must have a kind of accidental causation, which we might associate with chance. That is, there is no necessary reason why a musical man is musical, but rather it just so happens by chance that he is musical. Being as truth covers judgments that a given proposition is true. These sorts of judgments involve mental acts, so being as truth is an affection of the mind and not a kind of being in the world. Because accidental being is random and being as truth is only mental, they fall outside the realm of philosophy, which deals with more fundamental kinds of being.
The first five books of the Metaphysics jump around a great deal, and what ultimately emerges is a hodgepodge preparation for the investigation of substance that follows in books Zeta and Eta. Aristotle himself never uses the word metaphysics to describe his enterprise (the word was invented by a later editor and literally signifies nothing more than the books “after the Physics”), and it is not likely that he arranged for the various books of the Metaphysics to be grouped together. We should not be surprised, then, to find, for example, a series of unresolved puzzles in book Beta, only some of which are addressed later in the Metaphysics, or a set of definitions in book Delta, only some of which are used later in the Metaphysics. At some points, Aristotle seems to claim that his primary interest is “first principles,” at others he seems fundamentally interested in logic, and at one point he equates metaphysics with theology. All six books, however, set out to find the best approach to the truly fundamental questions of philosophy. Without these preliminary attempts, the stage would not properly be set for the investigation of substance that follows.
Metaphysics is not unique in that it studies being—after all, almost every field of study is interested in things that exist—but rather that it studies being qua being. The word qua is a Latin term often used by philosophers, and it means something like “in its capacity as.” For example, there are many different ways we could study humans. Biologists study humans in their capacity as living organisms, psychologists study humans in their capacity as beings with minds and consciousness, and anthropologists study humans in their capacity as social beings. A metaphysician, by contrast, would study humans in their capacity as beings that exist. That is, metaphysics is not so much interested in the different facts about existent entities as it is in the fact that these entities exist at all. What is it, metaphysics asks, that characterizes being itself? Aristotle says that this investigation is a search into first principles and causes. That is, metaphysics investigates the reason that there should be being at all, whereas the other sciences study the reasons behind various manifestations of being.