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It has often been supposed, however, that the argument from illusion suffices to refute all forms of perceptual direct realism. The argument from illusion is actually a family of different arguments rather than one argument. Perhaps the most familiar argument in this family begins by noting that objects appear differently to different observers, and even to the same observers on different occasions or in different circumstances. For example, a round dish may appear round to a person viewing it from directly above and elliptical to another viewing it from one side. As one changes position the dish will appear to have still different shapes, more and more elliptical in some cases, closer and closer to round in others. In each such case, it is argued, the observer directly sees an entity with that apparent shape. Thus, when the dish appears elliptical, the observer is said to see directly something which is elliptical. Certainly this elliptical entity is not the top surface of the dish, since that is round. This elliptical entity, a sensum, is thought to be wholly distinct from the dish.


In seeing the dish from straight above it appears round and it might be thought that then directly sees the dish rather than a sensum. But here too, it relatively sett in: The dish will appear different in size as one is placed at different distances from the dish. So even if in all of these cases the dish appears round, it will; also, appear to have many different diameters. Hence, in these cases as well, the observer is said to directly see some sensum, and not the dish.

This argument concerning the dish can be generalized in two ways. First, more or less the same argument can be mounted for all other cases of seeing and across the full range of sensible qualities ~ textures and colours in addition to shapes and sizes. Second, one can utilize related relativity arguments for other sense modalities. With the argument thus completed, one will have reached the conclusion that all cases of non-hallucinatory perception, the observer directly perceives a sensum, and not an external physical object. Presumably in cases of hallucination a related result holds, so that one reaches the fully general result that in all cases of perceptual experience, what is directly perceived is a sensum or group of sensa, and not an external physical object, perceptual direct realism, therefore, is deemed false.

Yet, even if perceptual direct realism is refuted, this by itself does not generate a problem of the external world. We need to add that if no person ever directly perceives an external physical object, then no person ever gains immediate non-inferential knowledge of such objects. Armed with this additional premise, we can conclude that if there is knowledge of external objects, it is indirect and based upon immediate knowledge of sensa. We can then formulate the problem of the external world in another way:



Problems of the external world: can, secondly? ,