"Philosophers have been especially concerned with the problem of consciousness—for example, how to explain the redness of red or the painfulness of pain. This is a very thorny issue. The problem springs from the fact that the redness of red that I perceive so vividly cannot be precisely communicated to another human being, at least in the ordinary course of events. If you cannot describe the properties of a thing unambiguously, you are likely to have some difficulty trying to explain these properties in reductionist terms."