Full Idea
Because existence is a property of concepts the ontological argument for the existence of God breaks down.
Gist of Idea
Because existence is a property of concepts the ontological argument for God fails
Source
Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884], §53)
Book Reference
Frege,Gottlob: 'The Foundations of Arithmetic (Austin)', ed/tr. Austin,J.L. [Blackwell 1980], p.65
A Reaction
The point being that existence (like number) is not a property of actual things. His proposition sounds rather dubious. The concept of unicorns exists quite entertainingly; it is the failure of actual unicorns to exist that is so heartbreaking.