 The reason emerges if one considers the epistemology of perception. Some philosophers hold that some beliefs about physical objects are non-inferentially justified, while others hold that this is never so, and that justified beliefs about physical states of affairs are always justified via an inference to the best explanation that starts out from beliefs about one’s experiences.  But direct realists as much as indirect realists admit that there can be cases where a person would be justified in believing that a certain physical state of affairs obtained were it not for the fact that he has good evidence that he is hallucinating, or else subject to perceptual illusion.  Moreover, given evidence of the relevant sort, it makes no difference whether direct realism is true, or indirect realism: the belief in question is undermined to precisely the same extent in either case.