Perception and evidence

Abstract

Perception is a source of knowledge: by looking at a white cup on a desk, one can come to know that there is a white cup on a desk. Schellenberg’s character Percy is in such an agreeable situation, the ‘‘good case’’. Her hapless Hallie, on the other hand, is in the ‘‘bad case’’: she is hallucinating a white cup on a desk. (For maximum contrast we may take Hallie to be a lifelong victim of hallucination, waiving the usual externalist worries about whether this is genuinely possible.) We may suppose that Percy and Hallie share a very specific visual state that completely characterizes the character of Hallie’s hallucinatory experience. We can think of Percy’s total visual state as entailing this specific visual state that he and Hallie share, as well as entailing the state of seeing a white cup, a state that Hallie of course is not in. Because of the shared state, Percy’s total visual state is indiscriminable from Hallie’s, in the following sense: a ordinary perceiver who starts off in one state and then at time t enters the other state would be unable—absent collateral information—to tell that a change of state had occurred at t. Although their states are indiscriminable, Hallie’s epistemic situation is significantly worse than Percy’s. Percy knows that there is a white cup on a desk; Hallie doesn’t. She does, like Percy, believe that there is a white cup on a desk, but this belief is false.