Here is an outline and exposition of what is often called the “Vertical Cosomological Argument” based on Thomas Aquinas’ thoughts in On Being and Essence.

Outline

A contingent being exists.
- Say, me – or the one who says nothing exists. 🙂
This contingent being must have a cause of its existence that is something other than the contingent being itself.
- A contingent being is one that exists but does not have to simply by virtue of what it is (its essence).
  - E.G., A triangle is a three sided, 2 dimensional, enclosed geometric figure.
  - This would hold even if no triangle actually existed.
  - Therefore “existence” is not part of the essence of a triangle.
- A thing that does not include “existence” in its definition does not have existence as its essence and must get it from something else.
  - Contingent things cannot be uncaused or else they would simply “be” (necessary being).
  - A thing cannot cause itself, for it must exist in order to cause.
Additional contingent beings cannot provide an adequate causal account for the existence of a contingent being.
- Otherwise one would have an infinite regress of contingent beings – none of which can account for their own being.
- Contingent beings cannot give each other existence in a great circle of being because nothing would account for the circle itself.
Therefore, a necessary being (a being that cannot not-exist) exists.
A necessary being’s essence would be existence.
- This is true for the same reason that a triangle does not have existence as part of its essence.
- Since this necessary being must exist to be what it is, existence must be its essence.
Since existence is limited by essences, this existence would be infinite.
- Essences limit existence by being what they are and not another.
  - E.G. A horse is not a tree because they have different natures.
- Since this essence is existence there is no limit to it.
An infinitely existing necessary being is what is meant by God.

Exposition

First we need to understand two terms: contingent and necessary. A contingent thing is something that exists but does not have to exist. Contingent things must be caused to exist because if they do not have to exist they must have a reason for existing outside of themselves. A necessary thing must exist because that is its nature.

The argument becomes clearer when we grasp the difference between what something is and whether something exists. When we define something we are telling what it is – but not that it actually exists. A thing can be defined that does not really exist – like the Easter Bunny or Tooth Fairy. But just because we know what something is, that does not mean it actually exists. If we did not know what something was, we could never know if it actually existed. So “what something is” and “that something is” must really be two different things. If this is true, then just because something has a definition does not mean that it exists (at least outside our imagination). To look at the same truth another way, if all triangles ceased to exist that fact would not change the definition of a triangle.

What all this means is that for something to exist it must be made to exist – it doesn’t exist just because someone defines it. Imagine if I was defining the Easter Bunny. I might say, “The Easter Bunny is a magical rabbit that hops around on Easter morning and hides eggs for children to find.” Because you know something about reality you would probably guess that this creature does not exist. You might say, “Oh, you made that up – a creature like that cannot exist.” But suppose I then said, “OK then, how about this: The Easter Bunny is a magical rabbit that hops around on Easter morning and hides eggs for children to find, and he actually exists.” Would my addition of “and he actually exists” at the end of the definition change anything? Of course not. We cannot simply define something into existence.

Adding “and it exists” to any thing’s definition does not make it the case that the thing actually exists any more than adding “and it exists” to the definition of a unicorn would mean that unicorns really exist.

If existence was part of a thing’s essence then it would have to exist by necessity – otherwise it would not be what it was. Thus, for all unnecessary things there is a difference between “what it is” and “that it is.” And that means that if it exists, something else is making it exist. Since this is true of everything in the universe something outside the universe is required to make it exist right now. This thing cannot be getting its existence from something else or we would be right back where we started from – with things that need something else to make them exist. Therefore, the existence of things that do not exist of necessity shows that there must be something that does exist of necessity – something that has never not-existed, nor can ever stop existing.

Further, this thing would have to have existence as its very nature. That is, necessary existence cannot have been given to a nature for that would make the thing contingent on whatever gave it its existence. To have existence as a thing’s nature means that existence is what it is. What this means is that this existence is unlimited, because a thing’s nature is what limits its existence. For example, a man’s being is limited to his “man-ness.” His existence does not extend to anything not in the nature of man (he is not also a horse or a tree or a rock).

But if a thing is existence itself, then it has no limiting nature – infinite existence. That thing, of course, is God.