Full Idea
Satan cannot exist because he is the most imperfect conceivable being, and existence is one of the perfections.
Gist of Idea
If Satan is the most imperfect conceivable being, he must have non-existence
Source
Colin McGinn (Logical Properties [2000], Ch.2)
Book Reference
McGinn,Colin: 'Logical Properties' [OUP 2003], p.49
A Reaction
The logic of this seems right to me. Presumably the theologians would hastily deny this as a definition of Satan; he must have some positive qualities (like power) in order to enact his supreme moral imperfections. NIce, though.