 First, it has been argued that the contents of perception exhibit certain features that cannot be exhibited by the contents of the propositional attitudes. In particular, perception is capable of representing impossible or contradictory states of affairs, as is the case when viewing certain Escher drawings and when undergoing various perceptual illusions. In contrast, it is argued that conceptual content must be consistent—one cannot have a belief with a contradictory content. Thus, Crane argues that reflecting on the waterfall illusion shows that the contents of perception cannot be conceptual (Crane 1988a). Since one cannot undergo a state with contradictory conceptual contents, and yet the waterfall illusion is a perceptual state with contradictory content—the object perceived seems to be both moving and not moving—it cannot be the case that perception involves conceptual contents (for more on this argument see, e.g., Mellor 1988, Crane 1988b, Crane 1992, and Gunther 2001).