Full Idea
A perceptual belief that p can have as its main cause the fact that p. More general facts (all men are mortal; e=mc2) cannot be the main cause of my belief, even if they do function causally in some way.
Gist of Idea
Perceptual beliefs may be directly caused, but generalisations can't be
Source
Jonathan Dancy (Intro to Contemporary Epistemology [1985], 11.5)
Book Reference
Dancy,Jonathan: 'Introduction to Contemporary Epistemology' [Blackwell 1985], p.178
A Reaction
Note that the perceptual belief can be the "main" cause; it seems to me that most beliefs are caused by judgements, though I may normally accept beliefs which are directly caused by perception, if I have no reason to challenge them.