Philosophical theories of perception are generally admitted to be responses to certain problems or puzzles allied to the ancient dichotomy between Appearance and Reality. For they have been mainly provoked by the incompatibility of the common–sense assumption that an external, physical world exists and is revealed to the senses with the well–known facts of perceptual variation and error. If only what is real were perceived just as if only what is right were done it is possible that many of those questions would never have been asked which lead to moral philosophy and a metaphysics of the external world. But sense perceptions of the same object vary so that it appears to have contradictory qualities and are sometimes completely deceptive. Nor do illusory differ internally from veridical perceptions. Moreover, perceptual variation and error can be unmasked only by such procedures as looking more carefully, listening harder, trying to touch, asking others, in short by more sense experience. So the senses are, as it were, both accused and judge in these disputes and why should a venal judge be trusted more than the criminal he tries? Such “correction” of one experience by another of the same kind seems no more reliable than the original “error.” Philosophers have found all this very puzzling.