Full Idea
The fact that all causal and representative theories of perception treat material things as if they were unobservable entities entitles us to rule them out a priori.
Gist of Idea
Causal and representative theories of perception are wrong as they refer to unobservables
Source
A.J. Ayer (Language,Truth and Logic [1936], Ch.2)
Book Reference
Ayer,A.J.: 'Language, Truth and Logic' [Penguin 1974], p.71
A Reaction
It seems to me that we can accept a causal/representative account of perception if we think of it in terms of 'best explanation' rather than observables. Explanation requires speculation, which logical positivists can't cope with.