This chapter outlines the theory of perception and presents insofar as they are separable, the reasons for realism. Existing theories of perception begin with the unquestioned assumption that it is based on sensations, and then go on to postulate some kind of operation that must occur to convert them into percepts. All theories of sensation-based perception imply a categorical distinction between perception and memory, the former depending on present stimulation and the latter on a retrieval of the traces of past stimulation. The theory of information-based perception avoids the difficulties by assuming that neither sensations nor images are entailed in having knowledge of the world. Both the psychology of perception and the philosophy of perception seem to show a new face when the process is considered at its own level, distinct from that of sensation. J. W. Yolton says that the information acquired in perception is “frequently sensory.”