You also might want to try picking up a translation of the De Principiis Natura (On the Principles of Nature.)
Aquinas wrote this in his earlier years, and it explains things like:
Substance, accident, subject, form, matter, privation, etc. (yes there is a subtle distinction between substance and subject that I was unaware of).
Also, read the Categories of Aristotle. It’s a short work, (though quite difficult) but it helps with the terminology (if only all of us were like St. Augustine who claims to have read it without difficulty.)
Brief primer of terminology that troubled me when I started reading Aquinas (others on here, feel free to correct mistakes)
Substance: think of this etymologically as “stands-under.” Aristotle defines it a number of ways and I poured through his metaphysics studying and re-studying what he meant to figure it out. I was always troubled by questions like “can something immaterial be a substance?” because of the way that we use the term in modern English. Obviously, it can as in “separate substance” (angel). What is a substance? That which “stands-under” all the accidents. (Actually, there was an attempt, I think in the 16th or 17th Century to change “substance” in English translations to “understands,” but obviously, this caused much confusion so they stayed with substance. In German it is “unterstehen.”)
Accident: Again, the etymology of this term helps as “falls-after.” Merely think of this as attributes of a thing. Brown hair, white skin, short tongue, etc. are all accidents. Accidents are what you perceive with your five senses.
Act: This is the “actuality” of a thing. Often times, Aquinas uses this term to coincide with “perfection,” which is something’s “fullness of being.”
Potency: I was taught by my Thomist Latin teacher that a better translation of “potentia” would be “ability.” Potency, essentially, is what something can be, or has the ability to be or do. The standard example is the acorn. An acorn is in potency to be an oak tree in act.
Quiddity: Again, etymology helps. This is the “whatness” of a thing. This term is interchangeable with “essence.”
Principle: Starting point.
Nature: The internal principle of motion within a thing.
Violence: An external principle of motion.
Essence: Again, the “whatness” of a thing. One has to be careful, but you are safe to say that the essence and the quiddity correspond to the definition of the thing. To truly understand what is meant by this term you have to read “De Ente et Essentia,” which is a mine field in itself. Think of essence in this way: when you see 3 men standing next to each other, you have a multiplicity which shares some commonalities – the commonality “man.” This is the essence, (although, in actuality, the term ‘man’ is a predication of the essence perceived, but you are safe to think of it this way.)
Prime matter: Matter as it exists in pure potency. Prime matter does not actually exist, but is a conception of what matter is. That is, all matter is informed in some way, but matter in itself is pure potency – this is prime matter.
Form: actuality of a “substrate” (usually of matter.) You can think of matter as a “substrate” and form as a “determination.”
Being: The act of existence. All things possess being. Just as someone is running by having the act of running, so something exists by having existence (habens esse)
As to the four causes, read the 2nd book of Aristotle’s Physics, but briefly:
Efficient cause: Moving cause.
Final cause: “That for the sake of which.” The end that things aim at.
Material cause: The matter that is the cause of a thing. Aristotle’s example is the bronze that a statue is carved out of.
Formal cause: By Aristotle’s definition, it is basically the “shape” of a thing. However, it is also the actuality of a thing.
Hope this helps. Aquinas is tough, but when you can get through the technical language, you will never find a greater philosopher, for both his precision and clarity. Behind the dry, dusty language is a mind on fire, and no genius in history rivals him in history (as far as philosophy is concerned.) Keep at it, he is worth it, and I guarantee there will be numerous “ah-ha” moments while you read him. To think, he claimed after his mystical experience, “all that I have written is but straw” is simply wonderous what God must be. I take that as a metaphor: his writing is like the straw that surrounds the kernal that is God.
Good luck!
