Thomas Aquinas, On Being and Essence. Trans. Armand Maurier. Toronto: Pontifical Institute of Mediaeval Studies, 1968.
It’s hard to know where to start with Thomas Aquinas. His various Summas are important, but that is a steep learning curve and also demands a good, working knowledge of secondary Thomist literature. I think this current volume might be the best place to start. It is short and focused, whereas the Summa is close to being a transcript of an oral Master’s thesis. The introduction by Armand Maurier makes this volume doubly good.
The argument at its most basic: being as esse is the actuality of being, the act of existence. Think of it along more dynamic lines. Being as ens is the substance that has the actuality of being. Whatever esse an ens has, the esse structures the ens.
Citations from this book will be by chapter and section. “1.1” is chapter one, section one.
“Essence must be something common to all the natures through which different beings are placed in different generas and species” (Aquinas 1.3). Neither form nor matter is an essence. Matter cannot be an essence because it is not a principle of knowledge (2.1). It can’t be either because both are needed for the being of a substance.
Key ideas: “The genus, then, signifies indeterminately everything in the species and not the matter alone” (2.8). It is the “whole” of a thing without its “this-ness,” or specific form. A genus is proportionate to the whatness of a thing; specifies to its form; and difference to the composite nature (2.9).
Human nature isn’t the form itself or the matter itself, and if we can’t know it through the matter, how do we know it? Human nature has its being in the intellect abstracted “all individuating factors” (3.6).
We predicate something when our intellect combines and divides things (3.8).
A form is only intelligible when it is abstracted from matter, and only something immaterial can abstract it.
Key idea: a being is either uncaused, caused by the principles of its being, or receives its being from outside itself. Therefore, everything whose being is distinct from its nature receives its being from another (4.7). Thomas suggests, though doesn’t really develop it, that one could extend this chain to the first cause, pure being.
You don’t have to agree with Thomas Aquinas. I am more of a Scotist myself when it comes to knowledge of God and the human will. But if you are unfamiliar with the arguments in this book, and you choose to criticize Thomas Aquinas, then you deserve something like what happened when Ed Feser reviewed Jeffrey Johnson’s book on Aquinas.