Full Idea
Abstractionists are unaware of the difficulty with relations - that they neither exist nor can be observed apart from the converse relation, the two being indivisible, as in grasping 'to the left of' and 'to the right of'.
Gist of Idea
We cannot learn relations by abstraction, because their converse must be learned too
Source
Peter Geach (Mental Acts: their content and their objects [1957], §9)
Book Reference
Geach,Peter: 'Mental Acts: Their content and their objects' [RKP 1971], p.33
A Reaction
It is hard to see how a rival account such as platonism could help. It seems obvious to me that 'right' and 'left' would be quite meaningless without some experience of things in space, including an orientation to them.