The First Vatican Council taught that the existence of God can be proven by our reason alone:
God, the origin and end of all things, can be known with certainty by the natural light of human reason, through the things that he created. (Dei Filius 2)
But the Church has never offered an actual proof of God; it has left that to the philosophers. Although many have attempted to prove God’s existence, what they end up with is mere arguments. They may be quite persuasive, but they lack the metaphysical certitude of a mathematical proof. They may presuppose some bit of knowledge, or they may leave room for possible doubt.
But the medieval understanding of God, which St. Thomas Aquinas espoused, does not allow for doubting his existence. The proof that follows is a paraphrasing of the Angelic Doctor’s many writings that dealt with this subject. It proves the existence of a being that is one, immutable, eternal, infinite, omniscient, and omnipotent.
In fact, you can be more certain that God exists than that you are reading this article right now.
A Brain in a Vat
Let’s start by taking a position of radical doubt. Suppose for a moment that you are not really a human being with an actual body. In reality, you are nothing more than a brain floating in a vat of fluids, with electrodes attached to various parts of your exterior that allow evil scientists to manipulate you into thinking that what you perceive is actually there, when in fact it is nothing more than an imaginary world constructed by the scientists. Right now, they are making you think that you are reading this article when in fact you are not.
From this point of extreme skepticism, we will prove beyond all possible doubt that God exists.
1. One cannot deny one’s own existence.
Cogito, ergo sum. Even if you’re just a brain in a vat, your own existence can be verified simply by the fact that you perceive—that is, you see, hear, smell, taste and touch things. Whether or not your perceptions are accurate is another question, but even if you doubt your own existence, you must exist, for it is impossible for a non-existent thing to doubt. In fact, the very act of doubting proves that you exist. Therefore, denying your own existence is a contradiction in terms. I can deny yours and you can deny mine, but I can’t doubt mine, nor can you doubt yours.
2. There is at least one thing that exists.
It is possible for you to be deceived in your perception. In fact, it’s conceivable that every one of your perceptions is a delusion. But even if that is the case—even if nothing you think exists actually exists—you still must exist.
Entity is the word we have for anything that exists. You exist, so you are an entity.
3. There is such a thing as existence.
You can know with certainty that there is at least one entity, at least one thing of which the term existence can be predicated. If there were no such thing as existence, nothing would exist, not even you. But, as we have seen already, that is impossible.
As Aquinas would say, there must be an “act of being” in which all entities participate. This act of being must itself exist; it must be an entity. Thomas calls this entity esse, which is Latin for “to be” or “to exist.”
4. The nature of esse is actuality.
Now that we have established that esse is an entity, we must ask: What is the nature of this entity? What is its definition?
To answer these questions, we must consider existence by itself, apart from everything else.
What do we mean when we say that something exists? We mean that it is actual. For example, an acorn is actually an acorn and potentially a tree. A tree is actually a tree and potentially lumber. Lumber is actually lumber and potentially a desk. A desk is actually a desk and potentially firewood. Firewood is actually firewood and potentially ashes.
In other words, a thing is actually what it is right now; it is potentially what it might be in the future.
Now when we say that something exists, we normally refer to actuality rather than potentiality. For instance, if I held up an egg and said, “This egg exists,” you would understand me, because what I am saying is “This egg is actual” or “This is actually an egg.” But if I held up the egg and said, “This chicken exists,” that would not make sense to you, because even though the egg is potentially a chicken (that is, the chicken exists potentially), the concept of existence applies primarily to the egg’s actual state and only secondarily to its potential state.
Now potentiality is still a form of existence, but we realize that it is, in some sense, inferior to actuality. In other words, potentiality is a “shade” of existence the same way that pink is a shade of red. Just as we would say that pink lemonade is red but not in the same way that Hawaiian punch is red, so we say that potentiality exists but not as much as actuality does. Actuality is the fullness of existence.
So, again, taking the brain-in-a-vat hypothesis, you know that you are actual, even if nothing else you perceive exists.
5. Esse is nothing but pure actuality.
Potentiality is a privation of actuality. That is, it is not a thing in itself but the absence of something. In the same way, darkness is not a substance itself but the absence (or privation) of light.
Now a thing considered in itself contains nothing but its fullness. The nature (or essence) of light consists of nothing but light itself; it does not contain darkness. Therefore, the essence of esse contains nothing but its fullness, actuality. There is no potentiality in the nature of esse. Thus, the essence of esse is pure actuality, just as the essence of light is pure light.
Thomas argues that all entities participate in esse insofar as they are actual. Therefore, that in which they participate—esse—must be actual. In fact, it cannot admit of any potentiality.
6. Esse not only does exist but must exist.
Existence itself is pure actuality, with no potentiality in it. This means that the essence of existence is nothing other than existence. Existence is its own essence.
From this it follows that esse itself must exist, for if it did not, it would violate its own essence, which is impossible.
7. Esse is distinct from everything else that exists.
You can know from step 1 that you exist, and we know from step 3 that esse exists. But we also know that the two are not identical.
Let’s say you’re just a brain in a vat, that everything you perceive is an illusion. You can still recognize that, while you are actual in some ways, you are potential in other ways. You actually perceive that you’re reading this article right now; you’re potentially perceiving something else. You are actually existing right now; you potentially exist five minutes from now. Moreover, anything else that may exist has the same attribute: Its essence is composed of both actuality and potentiality.
But, as we saw in step 5, esse is nothing but pure actuality. Thus, it must be distinct from any other entity.
8. Esse must be one.
If there were more than one esse, then there would be distinctions among them. But distinctions imply limitations, and limitations imply potentiality. But since esse is pure actuality, it has no limitations, which means there is no distinction in esse. Therefore, there is only one esse.
9. Esse must be immutable.
Change involves potentiality. In order for something to change, it must first have the potential to change; it must have a potentiality that is to be actualized. But since esse is purely actual, it has no potential to change. Therefore, esse is unchanging.
10. Esse must be eternal.
Time is nothing but the passing of the future into the present into the past. It is the changing of the not-yet into the now into the no-longer. But because esse does not change, it does not change from the future to the present to the past. It must be outside the realm of time, which means that there is no future, present, or past with esse. In other words, esse is non-temporal, or eternal.
11. Esse must be infinite.
Space is nothing but the changing of the over-here to the over-there. Anything that is actually here is potentially there. But because esse is immutable, it must be outside the realm of space. It has no spatial constraints—that is, esse is infinite.
12. Esse must be omniscient.
Even if you’re a brain in a vat, you can perceive that you have the capacity to know. Because you are only partly actual, and esse is purely actual, esse must know all there is to know. That is, esse is all-knowing, or omniscient.
13. Esse must be omnipotent.
You can perceive that you have the capacity to do some things that are logically possible. Since you are only partly actual, and esse is purely actual, esse must be able to do all things that are logically possible. That is, esse is all-powerful, or omnipotent.
We have thus proven the existence of a being (esse) that not only does exist but must exist and is one, unchanging, eternal, infinite, omniscient, and omnipotent. This matches our definition of God that we stated at the beginning.
We can conclude, then, that even if all of your sense perceptions are false, even if you are nothing but a brain in a vat being manipulated by scientists into believing that you are reading this article right now when in fact you are not, there are two things you can know with absolute, 100 percent certainty: (1) You exist, and (2) God exists.