Full Idea
Part of Frege's profound new idea of identity is that the criteria for identity of objects of a given kind is not a consequence of the way that kind of object is characterised, but has to be expressly stipulated as part of that characterisation.
Gist of Idea
Identity between objects is not a consequence of identity, but part of what 'identity' means
Source
report of Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884]) by Michael Dummett - Frege philosophy of mathematics Ch.13
Book Reference
Dummett,Michael: 'Frege: philosophy of mathematics' [Duckworth 1991], p.162
A Reaction
This makes identity a relative concept, rather than an instrinsic concept. Does a unique object have an identity? Do properties have intrinsic identity conditions, making them usable to identify two objects. Deep waters. Has Frege muddied them?