Full Idea
Anselm's first proof fails, because he treats existence as being a perfection, which it isn't, because that would make it a real predicate.
Gist of Idea
Anselm's first proof fails because existence isn't a real predicate, so it can't be a perfection
Source
comment on Anselm (Proslogion [1090], Ch 2) by Norman Malcolm - Anselm's Argument Sect I
Book Reference
'The Existence of God', ed/tr. Hick,John [Macmillan 1964], p.51
A Reaction
Not everyone accepts Kant's claim that existence cannot be a predicate. They all seem to know what a perfection is. Can the Mona Lisa (an object) not be a perfection? Must it be broken down into perfect predicates?