I have always thought Anselm’s ontological argument for God quite elegant and convincing:
1. God is that than which nothing greater can be conceived
2. God may exist in the understanding.
3. To exist in reality and in the understanding is greater than to exist in the understanding alone.
4. Therefore, God exists in reality.
Those who wish to resist the idea of there being a God declare this to be a circular argument; ironically, the same people have little difficulty accepting the Anthropic Principle, which really is circular:
The Universe (and hence the fundamental parameters on which it depends) must be such as to admit the creation of observers within it at some stage. (this is a version of the Strong Anthropic Principle. There are others, but they are all equally tautological)
Bertrand Russell, having not made out too well in his debate with Frederick C. Copleston, interviews Anselm who drags in Rowan as an example of something more obscure and intrinsically pleonastic than both the ontological argument and anthropic principle combined. Read it all here:
BR: Thank you for being here, Bishop.
Anselm: Glad to be here. Glad to be anywhere after all this time.
BR: Just a few preliminaries: You are the author of this treatise, called Proslogion?
Anselm: Why yes. It’s my best work. Proslogium, please. And I never liked Professor Kant calling my argument “ontological”—it was never called that in my day.
BR: Oh, and what was in called in your day?
Anselm: Anselm’s Argument.
BR: I see. And in this treatise you propose what you call an air-tight and foolproof argument for the existence of God?
BR: It’s all very…obscure, isn’t it?
Anselm: You think this is obscure? Thank God we’ll both be dead when Rowan Williams sits in my chair.