Full Idea
It was Frege who first made identity a logical notion, enshrining it above all in the formula (x) x=x.
Clarification
Gottlob Frege was an Austrian logician who flourished c. 1890
Gist of Idea
Frege made identity a logical notion, enshrined above all in the formula 'for all x, x=x'
Source
report of Gottlob Frege (works [1890]) by José A. Benardete - Metaphysics: the logical approach Ch.9
Book Reference
Benardete,José A.: 'Metaphysics: The Logical Approach' [OUP 1989], p.55