Full Idea
Husserl says the only ground for assuming the replaceability of one content by another is their identity; we are therefore not entitled to define their identity as consisting in their replaceability.
Gist of Idea
Content is replaceable if identical, so replaceability can't define identity
Source
report of Michael Dummett (Frege philosophy of mathematics [1991]) by Michael Dummett - Frege philosophy of mathematics Ch.12
Book Reference
Dummett,Michael: 'Frege: philosophy of mathematics' [Duckworth 1991], p.142
A Reaction
This is a direct challenge to Frege. Tricky to arbitrate, as it is an issue of conceptual priority. My intuition is with Husserl, but maybe the two are just benignly inerdefinable.