Full Idea
The sea is something real and a number is not; but this does not prevent it from being something objective; and that is the important thing.
Gist of Idea
Numbers are not real like the sea, but (crucially) they are still objective
Source
Gottlob Frege (Review of Husserl's 'Phil of Arithmetic' [1894], p.337)
Book Reference
-: 'Mind July 1972' [-], p.337
A Reaction
This seems a qualification of Frege's platonism. It is why people start talking about abstract items which 'subsist', instead of 'exist'. It shows Frege's motivation in all this, which is to secure logic and maths from the vagaries of psychology.