But here too you are wrong, since we must decide that
something which rests on a defect is unreal, although it may remain unrefuted.
Let us imagine a race of men afflicted with a certain special defect of vision,
without being aware of this defect, dwelling in some remote mountain caves
inaccessible to all other men provided with sound eyes. As we assume all of
these cave dwellers to be afflicted with the same defect of vision, they, all
of them, will equally see and judge bright things, e.