This chapter is concerned with questions about the forms and limits of our cognitive access to visual sensations, and with questions about the semantic properties of the concepts on which such access depends.

It is convenient to distinguish at the outset between two types of visual sensations. First, there are sensations that have the power to induce us to form perceptual beliefs about the visually perceptible properties of extramental objects and events. The members of this group, which will hereafter be called belief-generating sensations, include all of the sensations that occur in the course of everyday veridical visual perception. But the group has other members as well. Specifically, because we can be led to form perceptual beliefs – or, if you prefer, quasiperceptual beliefs – when we are dreaming and also when we are hallucinating, the group includes sensations that are involved in dreams and hallucinations. Second, there are visual sensations that lack the power to induce us to form perceptual beliefs. These sensations include after-images of various kinds and also the highly amorphous visual sensations that come to the fore when we close our eyes. I will refer to members of this second group as perceptually barren sensations.

In defining belief-generating sensations I said that they are sensations that have the power to generate perceptual beliefs.