Full Idea
Guanilo supposes that we imagine an island surpassing all lands in its fertility. We might then say that we cannot doubt that it truly exists is reality, because anyone can conceive it from a verbal description.
Gist of Idea
Guanilo says a supremely fertile island must exist, just because we can conceive it
Source
Anselm (Proslogion [1090], Reply 3)
Book Reference
'The Existence of God', ed/tr. Hick,John [Macmillan 1964], p.27
A Reaction
Guanilo was a very naughty monk, who must have had sleepless nights over this. One could further ask whether an island might have necessary existence. Anselm needs 'a being' to be a special category of thing.