Full Idea
Being is obviously not a real predicate, i.e. a concept of something that could add to the concept of a thing.
Clarification
A 'predicate' here means a property
Gist of Idea
Being is not a real predicate, that adds something to a concept
Source
Immanuel Kant (Critique of Pure Reason [1781], B626/A598)
Book Reference
Kant,Immanuel: 'Critique of Pure Reason', ed/tr. Guyer,P /Wood,A W [CUO 1998], p.567
A Reaction
Kant's famous slogan against the Ontological Argument. The modern line is that existence is a quantifier, which stands outside a proposition, and says whether it applies to anything. It is worth considering the possibility that Kant is wrong.