Full Idea
One powerful argument for a thesis that one notion is conceptually prior to another is the possibility of defining the first without reference to the second.
Gist of Idea
Maybe a concept is 'prior' to another if it can be defined without the second concept
Source
Michael Dummett (Frege philosophy of mathematics [1991], Ch.12)
Book Reference
Dummett,Michael: 'Frege: philosophy of mathematics' [Duckworth 1991], p.145
A Reaction
You'd better check whether you can't also define the second without reference to the first before you rank their priority. And maybe 'conceptual priority' is conceptually prior to 'definition' (i.e. definition needs a knowledge of priority). Help!