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Full Idea
Before we can conclude that φ expresses a sortal concept, we need to ensure that 'is the same φ as' generates statements of genuine identity rather than of some other equivalence relation.
Clarification
Equivalence relations are transitive, symmetric and transitive
Gist of Idea
A concept is only a sortal if it gives genuine identity
Source
Crispin Wright (Frege's Concept of Numbers as Objects [1983], 1.i)
Book Ref
Wright,Crispin: 'Frege's Conception of Numbers' [Scots Philosophical Monographs 1983], p.3
Related Idea
Idea 13865 'Sortal' concepts show kinds, use indefinite articles, and require grasping identities [Wright,C]