Full Idea
Radical behaviourists say animals emit "similar" responses to "similar" reinforcements, but that is empty without specifying in what respect there is a similarity.
Gist of Idea
How are stimuli and responses 'similar'?
Source
Georges Rey (Contemporary Philosophy of Mind [1997], 4.3)
Book Reference
Rey,Georges: 'Contemporary Philosophy of Mind' [Blackwell 1997], p.124
A Reaction
The point is that when you try to specify the similarity you are (supposedly) forced to use mental language to make the distinctions, thus contradicting behaviourism. It is not, though, self-evidently impossible to give a behaviourist specification.