What follows is the excerpt from my 2003 book (which is still available for purchase - open THIS LINK if you're interested). Happy Feast of St Thomas!
In a small early treatise entitled De Ente et Essentia, St Thomas proposes an argument for the existence of God which takes as its starting point not the various activities and characteristics of created things, but their most fundamental "activity" - their very act of existing. In chapter five of De Ente et Essentia, Thomas argues that, with regard to all of the things that we encounter, "what they are" cannot explain the fact that they actually do exist in reality. Therefore their existence must be explained in terms of Something Else; it must be caused by Something Else.
The main principle that St Thomas develops in this little treatise is that there is a real distinction between essence and existence, between the essentially integrated complexus of specifications that define what something is, and the actualization of that specificity, the "placing" of it into the real universe of existence.
The existing of something - its esse (to use the Latin infinitive for the verb "to be"), the act by which it "is" - is distinct from "what it is," its essence. Existing introduces another dimension beyond everything we can say about what a thing is. When I say "the horse exists," I take the whole richly envisioned description of "horse" that I an able to apprehend and examine with my mind, and I affirm "something else" with regard to the horse, something that is not part of the description of a horse; I judge an instance of this essence (the "whatness" of horse, "horseness") to be there in reality. The horse is. The horse is existing.
Notice something very important here: existing is an act. It is the most fundamental of all acts. To exist is dynamic, radically dynamic. The reason why we think of existing as static is because we are surrounded by it everywhere and therefore inclined to ignore it or regard it as commonplace. But things are not just plopped around us - just "there," with this fact worthy of nothing more than a "ho-hum" from our faculties of perception. On the contrary, things are bursting with being. "Is-ing" is a fascinating and powerful achievement - the achievement of really existing which should never be taken for granted, as something that does not provoke our minds to wonder (American philosopher Frederick Wilhelmsen was known for his vigorous presentation of this point in his famous "is-ing" lecture).
We have allowed ourselves to be lulled into metaphysical sleep by the apparently commonplace character of the existence of things. We must realize, instead, that the existing of any thing is a spectacular and awe-inspiring event.
This realization, achieved by means of a sufficiently intense attention to the reality of things, will lead us to recognize that existing is an actualization that comes to an essence "from outside." It is at one and the same time the fundamental act that any thing "does," and an act that does not emerge from a thing's own essential power, and which therefore must be brought about in it by Something Else. When we say "John runs," we recognize that "running" is an activity distinct from "John," something that John does; John "actualized" in a certain respect, John moving from one place to another. However, this act takes place as a result of John's own inherent capacities: he causes himself to run by means of the muscular energy he possesses by virtue of the organic and "animal" characteristics that are proper features of his essence.
When we say "John exists," however, we are talking about the fundamental actualization of John, Could John be the cause of his own existence? It's impossible. In order to bring about an effect, a cause has to "be there," but John without the act of existing is not "there," and therefore he cannot bring about any effect at all, much less his own existence.
So how is it, then, that John exercises the act of existing? He must receive the impetus of this act from some outside source. At this point in the argument, St. Thomas invokes the principle that there cannot be an infinite series of caused causes, and thus concludes that there must be a First Cause that gives to all other things their act of existing. As First Cause, it is the Origin of all existence, which means that it does not receive its act of existing from anywhere else. It possesses "existing" properly, as the very definition of what it is. Its essence is "to exist; it is Sheer Existence, the subsistent, pure act of "to be." This Being, indeed, is the Being we call God.
To summarize the argument in simple terms: Our existence does not come from ourselves; it is given to us. This means that there must be a Giver of existence who is the Source of this gift, who therefore possesses it essentially and fully. We have existence because we receive it from the One who is Existence - Pure and Absolute Existence.
