Full Idea
Clearly that than which a greater cannot be thought cannot exist in the understanding alone. For it it is actually in the understanding alone, it can be thought of as existing also in reality, and this is greater.
Gist of Idea
If that than which a greater cannot be thought actually exists, that is greater than the mere idea
Source
Anselm (Proslogion [1090], Ch 2)
Book Reference
'The Existence of God', ed/tr. Hick,John [Macmillan 1964], p.26
A Reaction
The suppressed premise is 'something actually existing is greater than the mere conception of it'. As it stands this is wrong. I can imagine a supreme evil. But see Idea 21243.
Related Ideas
Idea 21243 An existing thing is even greater if its non-existence is inconceivable [Anselm]
Idea 21241 Even the fool can hold 'a being than which none greater exists' in his understanding [Anselm]