Full Idea
Behaviourists refuse to admit images because they cannot be observed from without, but this causes them difficulties when they attempt to explain either memory or imagination.
Gist of Idea
Behaviourists struggle to explain memory and imagination, because they won't admit images
Source
Bertrand Russell (My Philosophical Development [1959], Ch.13)
Book Reference
Russell,Bertrand: 'My Philosophical Development' [Routledge 1993], p.111
A Reaction
This is a striking objection to behaviourism, and it is rarely mentioned in modern discussions of the topic. They might try denying the existence of private 'images', but that wouldn't be very plausible.