Full Idea
Indispensable to cross-reference, lacking distinctive content, and pervading thought and discourse, 'identity' is without question a logical concept. Adding it to predicate calculus significantly increases the number and variety of inferences possible.
Gist of Idea
Identity is clearly a logical concept, and greatly enhances predicate calculus
Source
George Boolos (To be is to be the value of a variable.. [1984], p.54)
Book Reference
Boolos,George: 'Logic, Logic and Logic' [Harvard 1999], p.54
A Reaction
It is not at all clear to me that identity is a logical concept. Is 'existence' a logical concept? It seems to fit all of Boolos's criteria? I say that all he really means is that it is basic to thought, but I'm not sure it drives the reasoning process.