Full Idea
Number is neither spatial and physical, like Mill's pile of pebbles, nor yet subjective like ideas, but non-sensible and objective.
Gist of Idea
Numbers are not physical, and not ideas - they are objective and non-sensible
Source
Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884], §27)
Book Reference
Frege,Gottlob: 'The Foundations of Arithmetic (Austin)', ed/tr. Austin,J.L. [Blackwell 1980], p.38
A Reaction
This doesn't require commitment to full-blown universals, nor to a dualist world of mind. The thinking of the brain moves far away from the areas of sensation, and the brain's capacity for truth is its capacity for objectivity.