Here’s one example, as given by St. Thomas Aquinas:
Those things are said to be self-evident the truth of which is obvious once the meaning of the words is clear. For example, when we understand the meanings of the words "whole" and "part," we immediately realize that every whole is greater than its part. Once we understand the meaning of the word "God," however, it immediately follows that God exists. The word itself signifies "that being a greater than which cannot be signified." That which exists in fact and in the mind is greater than that which exists in the mind alone. Thus, since the moment we understand the meaning of the word "God" he exists in our minds, it follows that he must also exist in fact. Thus God's existence is self- evident.
This is basically St. Anselm’s proof—almost a thousand years old. For a time, it was generally felt that Kant had disproven it, but it is back in contention, as reformulated by Godel and Plantinga. The consensus among philosophers currently is apparently that it is sound, that nobody has found a fatal flaw.
It could be argued that "great" is not an entirely coherent concept; but no matter. The proof works as well if we substitute "important" or "real" or several other terms.