Full Idea
Spatial predicates are not applicable to ideas; an idea is neither to the right nor to the left of another idea; we cannot give the distances between ideas in millimetres.
Gist of Idea
Ideas are not spatial, and don't have distances between them
Source
Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884], §61)
Book Reference
Frege,Gottlob: 'The Foundations of Arithmetic (Austin)', ed/tr. Austin,J.L. [Blackwell 1980], p.72
A Reaction
This Fregean thought should be music to the ears of Cartesians, though it does not seem intended as support for dualism. This is the logicians' view of reality, where true inferences are what matter, and brains and souls are irrelevant.