Probably the single most famous “proof” in all of philosophy is refuted if the topic of our discussion is legitimate. In his Proslogion, St. Anselm lays out the following argument for the existence of God:
“And certainly that than which a greater cannot be imagined cannot be in the understanding alone. For if it is at least in the understanding alone, it can be imagined to be in reality too, which is greater. Therefore if that than which a greater cannot be imagined is in the understanding alone, that very thing than which a greater cannot be imagined is something than which a greater can be imagined. But certainly this cannot be. There exists, therefore, beyond doubt something than which a greater cannot be imagined, both in the understanding and in reality.”1
St. Thomas, of course, rejects this “proof” of St. Anselm in his Summa Theologica:
“Granted that everyone understands that by this word “God” is signified something than which nothing greater can be thought, nevertheless, it does not therefore follow that he understands that what the word signifies exists actually, but only that it exists mentally. Nor can it be argued that it actually exists, unless it be admitted that there actually exists something than which nothing greater can be thought; and this precisely is not admitted by those who hold that God does not exist.”2
The problem is basically this: is to ask the question “is it?” the same as to ask the question “what is it?” Or is one a question of concept and the other a question of judgment? For to say what something is, according to Aristotelian logic, is the understand an essence, but this is not the same thing as to affirm the existence of that essence.
The ontological proof offered by Anselm was certainly defended by Descartes, and his rationalist proof for the existence of God is very similar in its construction and assumptions. Even to say “I think, therefore I am” implies a somewhat related premise, but this is not the place to go further into that.
Of course, a refutation was made to Anselm, “on behalf of the fool,” where one simply asked about “imagining a perfect island” and then seeking to see how this would prove the existence of this island, and Anselm responded accordingly. This is, of course, not the place to enter into that particular dispute, but we see here that even the great thinkers of our Christian heritage differed vastly on this question, and it is one that ultimately comes down to the difference between essence and existence, or “being” in the “verb sense.”
According to St. Thomas, God is Ipsum esse subsistens (Subsistent Act of Existing Itself). If he is correct, God and God only is his existence. This means that in every other existing thing, essence and existence are different. And this will lead us to know that all other things are contingent, as they do not explain their own existence in their very essence. These said, it should be clear that the difference between essence and existence is of primary importance in metaphysics and in all contemplation of reality.