Full Idea
Because predicate logic contrues identities dyadically, its account of inferences involving identity propositions needs laws or axioms of identity, explicitly asserting that the dyadic realtion in 'x=y' possesses symmetry, reflexivity and transitivity.
Clarification
'Dyadic' means two-place
Gist of Idea
Predicate logic has to spell out that its identity relation '=' is an equivalent relation
Source
Fred Sommers (Intellectual Autobiography [2005], 'Syllogistic')
Book Reference
'The Old New Logic', ed/tr. Oderberg,David S. [MIT 2005], p.14