Full Idea
We cannot define number by the generalized concept of a series. Positions in the series cannot be the basis on which we distinguish the objects, since they must already have been distinguished somehow or other, for us to arrange them in a series.
Gist of Idea
We cannot define numbers from the idea of a series, because numbers must precede that
Source
Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884], §42)
Book Reference
Frege,Gottlob: 'The Foundations of Arithmetic (Austin)', ed/tr. Austin,J.L. [Blackwell 1980], p.54
A Reaction
You can arrange things in a line without the use of numbers. You need prior mastery of counting, though, to say where an item comes in the line. And yet... why shouldn't you define counting by the use of some original primitive line? Numbers map onto it.