The Ontological proof (Anselm's version):
- God is something than which nothing greater can be thought.
- It is greater to exist in reality and in the understanding than just in understanding.
- Therefore, God exists in reality
The problem being that 2. is clearly false. Many things exist in potentia or in theory, that would be less perfect if actualised - the Platonic Form of a carpenter's conception of a chair is perfect, the actualisation in wood is not, necessarily. The plan for a novel is always a greater thing than the imperfect realisation of it, actualised by the author's hand.
Thus, "something which is greater than all things" cannot but be a Platonic Form, rather than an actualisation, and hence cannot exist - for in existing, it must become less than it was in potential.
This is all sounding very gnostic, isn't it?