Metaphysics (Aristotle)
Books VII–IX: Zeta, Eta, and Theta

Books Zeta, Eta, and Theta are generally considered the core of the Metaphysics. Book Zeta (VII) begins by stating that "being" has several senses, the purpose of philosophy is to understand the primary kind of being, called substance (ousia) and determine what substances there are, a concept that Aristotle develops in the Categories. Zeta goes on to consider four candidates for substance: (i) the ' essence ' or 'what it is to be' of a thing (ii) the universal, (iii) the genus to which a substance belongs and (iv) the material substrate that underlies all the properties of a thing.

Finally, he concludes book Zeta by arguing that substance is really a cause.

Book Eta consists of a summary of what has been said so far (i.e., in Book Zeta) about substance, and adds a few further details regarding difference and unity.

Book Theta sets out to define potentiality and actuality. Chapters 1–5 discuss potentiality, the potential of something to change: potentiality is "a principle of change in another thing or in the thing itself qua other." In chapter 6 Aristotle turns to actuality. We can only know actuality through observation or "analogy;" thus "as that which builds is to that which is capable of building, so is that which is awake to that which is asleep...or that which is separated from matter to matter itself". Actuality is the completed state of something that had the potential to be completed. The relationship between actuality and potentiality can be thought of as the relationship between form and matter, but with the added aspect of time. Actuality and potentiality are distinctions that occur over time (diachronic), whereas form and matter are distinctions that can be made at fixed points in time (synchronic).
length_constraints:number_paragraphs[2]