Full Idea
'There is one and only one entity x which is most perfect; that one has all perfections; existence is a perfection; therefore that one exists' fails as a proof because there is no proof of the first premiss.
Gist of Idea
The ontological argument begins with an unproven claim that 'there exists an x..'
Source
Bertrand Russell (On Denoting [1905], p.54)
Book Reference
Russell,Bertrand: 'Logic and Knowledge', ed/tr. Marsh,Robert Charles [Routledge 1956], p.54
A Reaction
This is the modern move of saying that existence (which is 'not a predicate', according to Kant) is actually a quantifier, which isolates the existence claim being made about a variable with a bunch of predicates. McGinn denies Russell's claim.