by Scott F. Aikin and Robert B. Talisse
The Ontological Argument is an infamously devilish a priori argument for God's existence. It runs, roughly, as follows.
God is by definition is the greatest possible thing.
If God is the greatest possible thing, then He cannot fail to manifest any perfection — otherwise, there would be a possible thing greater than He.
Existence is a perfection; that which does not exist lacks something that would improve it.
Therefore, God must exist.
The conclusion can be strengthened, further, with the thought that necessary existence is a greater perfection than contingent existence, and so it is necessary that God necessarily exists. Now, that's a pretty heavy conclusion derived only from some strikingly lightweight premises. This is what makes the Ontological Argument so interesting – it seems clear that something's gone wrong, but it turns out that it's very hard to explain what it is.