Full Idea
The stipulation that the direction of a line a is to be the same as that of a line b just in case a is parallel to b does not determine whether the direction of a line is itself a line or something quite different.
Gist of Idea
Defining 'direction' by parallelism doesn't tell you whether direction is a line
Source
comment on Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884], §60) by Michael Dummett - Frege philosophy of mathematics Ch.11
Book Reference
Dummett,Michael: 'Frege: philosophy of mathematics' [Duckworth 1991], p.126
A Reaction
Nice point. Maybe not being able to say exactly what something is is either a symptom of nonsense, and simply a symptom that we are dealing with an abstract concept. If abstractions don't exist, they don't need individuation criteria.