Metaphysics - Aristotle, Richard Hope Metaphysics, true to its name, studies the essence of being. The study of “being qua being” seems to be the phrase of choice. Much of the book appears to be an extension of ideas originally brought forth in Categories. Essentially, Aristotle attempts to define the essence of a thing. What makes each thing…“be.”

Not surprisingly, Aristotle spends considerable time with definitions. He reacts to his predecessors, contemporaries and the absurdities in the sophist tradition.
And definition arises out of the necessity of stating what we mean; since the statement of which the word is a sign becomes a definition. Now the argument of Heraclitus, to the effect that all things are and are not, seems to present all things as true, whereas that of Anaxagoras, that there is an intermediate between contradictories, seems to present all things as false; for when things are mixed, the mixture is neither good nor not good, so that one cannot say anything true. pg. 84; Book Gamma
With mathematical precision, Aristotle then defines his terms, ultimately leading to the same effort to define essence. Aristotle concludes the essence of a thing is found in the whole that precedes its parts. Repeatedly, he comes back to the idea of man. For example, man may be musical but the music doesn’t define what a man is. But the constant division of substance into categories seems to yield the same problem Aristotle notes other philosophers encounter when they search for independent substance in numbers:
it was other philosophers who separated them from things and gave them the name “ideas”. And they concluded by practically the same reasoning that there is an idea for each general term; as if they thought it was easier to count many than few. pg. 277-278; Book Mu
Likewise, the constant division to reach a prior whole by unique identifiers ultimately leaves categorization meaningless. It seems impossible to identify essence through observation of traits.

As Aristotle struggles with distinguishing wholeness from “accidentals” (such as musicality, whiteness, etc.) to define man, he clearly is starting from a definite position that he doesn’t try to explain; why is man a whole and not an accidental? Maybe the essence of man is found in constituent parts. Maybe man is an accidental. Aristotle did not have the advantage of knowing genetics, or, in even more elemental form, the 118 elements of the periodic table. Most of Aristotle’s attempt at definition get stumped at what can be observed. Such science also must be taken into consideration in Book Mu and Book Nu where Aristotle disregards the independent essence of numbers. “How can whiteness be a ratio between numbers?” he asks at one point. In genetic terms, very easily. Of course, his questions of “being qua being” still apply to genes and elements, but his analysis obviously cannot reach such a stage.

The most enduring principles out of Metaphysics seem to be the idea of a prime mover and the principles of contradiction (a thing cannot be and not-be at the same time). The idea of prime mover, or first cause, I recall being a foundational principle of St. Thomas Aquinas and the Christian Neoplatonists, but I’ve still got about a millennia of works to get through before I get to them.