The scene: A hall at the Abbey of Bec, around 1080. A lecture is taking place. Presenting his ideas, the Abbot, later to be Saint Anselm of Canterbury.
Anselm: Let us contemplate that of which nothing greater can be conceived. Now, should this thing exist only in the intellect, it would not be that of which nothing greater can be conceived, since to exist in reality is greater than not to exist. It follows, then, that that of which nothing greater can be conceived must actually exist in reality. It is this which we call God.
Brother PiaTor, a rather lazy and smart-assed monk: But I can think of an Entity greater than your God.
Anselm: What? What is this you say, brother?
PiaTor: Well, it follows that your God cannot think of an entity greater than itself. All I must do is posit an Entity which has all the attributes of your God, but with one additional capability - It can conceive of a yet greater Being. It follows that my Entity, sharing in the attributes of your God, must also exist, and must be greater than your God.
PiaTor: Come to think of it, that greater Entity of which my Entity can conceive must also exist. Hey - we've just refuted monotheism. There's an infinite number of Gods, all all-perfect! This is - wait a minute! Put me down!
Anselm: We shall never speak of this again. Scribe, burn the record of the lesson.
Anonymous Brother: And the heretic, Lord Abbot?
Anselm: The rose garden is looking a bit under-nourished this season...