Esse est percipi, to be is to be perceived, said good old Berkeley; but, according to most philosophers, he was wrong. Yet, obviously, there are things for which the adage holds. Perception, trivially, to begin with. If elements of conscious awareness--pains, tickles, feelings of heat and cold, sensory qualia of colors, sounds, and the like--have any existence, it must consist in their being perceived by a subject.... This shows, of course, that such experiences are epiphenomenal, at least with respect to the physical world.