We normally regard seeing as intimately connected with light. But must seeing involve light? Suppose you could step into a pitch-dark room and have precisely the experiences you would have if it were fully lighted. The room would thus look to you just as it would if fully lighted, and you could find any unobscured object by looking around for it. Would this not show that you can see in the dark? If so, then the presence of light is not essential to seeing.

However, the case does not establish quite this much. For seeing is a causal relation, and for all I have said you are just vividly hallucinating precisely the right things rather than seeing them. But suppose you are not hallucinating and that if someone covered a coin you see with lead or covered your eyes, you would no longer have a visual experience of a coin. In this case, it could be that somehow the coin affects your eyes through a mechanism other than light transmission, yet requiring an unobstructed path between the object seen and your eyes. Now it begins to seem that you are seeing. You are responding visually to stimuli that causally affect your eyes. Yet their doing so does not depend on the presence of light

—Robert Audi

(This post was excerpted, with Audi’s permission, from his Epistemology: A Contemporary Introduction to the Theory of Knowledge. It’s an excellent example of how it is possible to make an interesting and even important philosophical point in very few words.)