Full Idea
Something can be thought of as existing, which cannot be thought of as not existing, and this is greater than that which cannot be thought of as not existing.
Gist of Idea
An existing thing is even greater if its non-existence is inconceivable
Source
Anselm (Proslogion [1090], Ch 3)
Book Reference
'The Existence of God', ed/tr. Hick,John [Macmillan 1964], p.26
A Reaction
This is a necessary addition, to single out the concept of God as special. But you really must give reasons for saying God's non-existence is inconceivable. Atheists seem to manage.
Related Ideas
Idea 21242 If that than which a greater cannot be thought actually exists, that is greater than the mere idea [Anselm]
Idea 21241 Even the fool can hold 'a being than which none greater exists' in his understanding [Anselm]