Full Idea
Whether a geometry can be applied to the actual physical world or not, is an empirical question which falls outside the scope of the geometry itself.
Gist of Idea
Whether geometry can be applied to reality is an empirical question outside of geometry
Source
A.J. Ayer (Language,Truth and Logic [1936], Ch.4)
Book Reference
Ayer,A.J.: 'Language, Truth and Logic' [Penguin 1974], p.110
A Reaction
This is a key objection to rationalism by empiricists. You may say that geometry applies to your car, but your car may have been pulverised while you were talking. Why, though, did Einstein find non-Euclidean geometry so useful?