It would be wrong of me to suppose that just because I can form private mental images, that everyone can. As Francis Galton and William James long ago showed, a small proportion of adults--and some of these extremely intelligent--are unable to form such visual images. Berkeley's point is that it would be equally arrogant for these non-thinkers or non-image formers to claim that everyone is like them in the relevant respect. The temptation to pontificate in that way reveals a narrowness and unwillingness to see the world from another perspective.
Source: Berkeley: The Great Philosophers (The Great Philosophers Series) (Great Philosophers (Routledge (Firm))), Pages: 49