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.
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