I’ve been doing posts on Thomas Aquinas, his life and contribution to the church. In this last post, I want to consider Thomas’ metaphysics. McInerny defines metaphysics as the “study [of] being as being, that is, whatever is just insofar as it is a being”.[1] This topic of being is quite obviously vast, and could consider humans, angels, matter, immaterial things, God, et al. For this reason, in this post, I want to try and consolidate Thomas’ teachings on metaphysics to the broadest categories. I want to consider where being originates.
Thomas followed the great Greek thinkers when considering metaphysics, especially Aristotle. Feser explains that during Aristotle’s time, a philosopher named Parmenides said that,
[C]hange [in being] is impossible. For a being could change only if caused to do so by something other than it. Hence, though the senses and common sense tell us that change occurs all the time, the intellect…reveals to us that they are flatly mistaken.[2]
Parmenides assumed that if there were a source of all being, it would have to be non-being, which of course is nothing. Aristotle strongly disagreed with Parmenides’ arguments. He contended instead that it was simply plain that everything could potentially change. He argued for this by making a distinction between what he called act and potency. By this he meant that everything has an act (what it actually is) and potency (potential to become something else). Thomas took this language and, while agreeing with Aristotle, argued that while everything has act and potency, nothing can simply actualize its own change. There must be something outside of it, moving or causing it to change. Feser explains:
An additional, external factor is required [to create change]. [Nothing can] actualize itself…As Aquinas says, “Potency does not raise itself to act; it must be raised to act by something that is in act”. This is the foundation of the famous Aristotelian-Thomistic principle that “whatever is moved is moved by another”[3]
Put another way, nothing in this universe becomes anything in and of itself. It is always caused by the something outside of itself. Or thinking of it another way, there is a great something outside of everything that actualizes all things.
Thomas continued in this line of argument by attributing this something outside of everything as God himself. God himself is the great thing that actualizes everything, but is himself not actualized. Thomas went on to clarify that when anyone attempts to attribute existence to something outside of it, that person is speaking, purposefully or not, of God. Here one arrives at Thomas’ great metaphysical teaching: God is being itself. Taylor Marshall explains:
There must be a “greatest” when it comes to “being” or “existence”. The greatest way of existing would be existence itself, and this we call “God”… God is being Himself. God is not caused or created, he just is. According to Thomas Aquinas, God is existence and everything else exists in God.[4]
So then God is being itself, and everything finds its source or cause from him. Aquinas confirmed this truth from the Exodus account of God’s name: I AM WHO I AM. God’s very nature is being itself. Or, put another way, God’s existence is his essence. Thomas famously called God “esse essentia” , Latin for existence is essence.
Based on this truth, Thomas explains that everything else finds its being in and cause from God. He is that great cause which actualizes everything. Taylor Marshall says that “God is existence in itself and…humans only participate in God’s existence”.[5] In this way then, God is the only being whose essence is existence itself. All other things have an essence, or nature, which owes its existence to God. Mankind has a human nature, which finds its source in God. Animals have their own natures, which owe their existence to God, and so on. To understand this teaching, one might think of Paul’s great sermon at Mars Hill in Acts 17:28. Paul says of mankind, that “in God we live and move and have our being”. This is at the basic level what Thomas is teaching: God is the uncreated source of all being.
Thomas went on to use this metaphysic to argue his famous five ways for proving God’s existence. He called the first the “argument from motion”, or change. The essential argument is that there are things that are in motion, and are changing and morphing. One may ask: from whence does this motion originate? Motion originates of course from something that causes it from the outside. However, there must be some ultimate cause, which is itself unmoved: Thomas argues this must be God. Thomas will go on to give four other arguments that logically proceed from the first. It was his opinion that this metaphysic demanded God, because he the source of all being itself.
[1] Ralph McInerny, Aquinas, 76
[2] Edward Feser, Aquinas, 9
[3] Ibid, 11
[4] Taylor Marshall, Thomas Aquinas in 50 Pages, 26
[5] Ibid, 27
Lucas Hattenberger 
You write, “In this way then, God is the only being whose essence is existence itself. All other things have an essence, or nature, which owes its existence to God. Mankind has a human nature, which finds its source in God. Animals have their own natures, which owe their existence to God, and so on.”
I just wanted to point out that St. Thomas did allow for secondary causes. His point was not necessarily that everything is directly created by God. It’s not incorrect to say that I received my being from my parents, for example, or that my being is sustained through food and water. What Thomas is saying is that you can’t take secondary causes to be the ultimate cause of everything. So for example, it’s possible that our natures arose via evolutionary processes; however evolutionary processes can’t be their own causes any more than you or I can. And while my physical being is sustained by food and water, those things don’t cause themselves either. Every cause that we know of is a secondary cause and therefore is itself caused. But you can’t go on tracing secondary causes forever. The train has to have an engine somewhere.
Yes, I was trying to be basic….he did argue for second causes. And yes, like you said, second causes eventually find their way to back to a first cause. Which, is again, God.