 Though Marr’s theory posits representational states, it does not make use of the notion of a visual experience, and it is an open question how experiences map onto the stages of computational visual processing posited by the theory (see Marr (1982), section 7.2 for Marr’s own brief discussion of this issue). Marr’s theory seeks to explain how the visual system arrives at representations of a three-dimensional array given retinal stimulation as input. The retinal stimulations, or ‘gray arrays’, are computational descriptions that represent light and intensity values in a two-dimensional coordinate system. According to Marr, the visual system computes a description of a three-dimensional scene in three stages. First, given the gray arrays, it computes a ‘primal sketch’ of reflectance changes on the surface outside the viewer. Next, with the primal sketch as input, it computes a ‘2.5-D sketch’ of this surface, a sketch that explicitly represents information concerning depth and contours of the surfaces relative to the viewer. (The depth information concerns only facing surfaces, so that the 2.5 sketch contains no information—and therefore no misinformation—about how surfaces continue out of view.) Finally, given the 2.5 sketch as input, the visual system computes information concerning all three dimensions of objects, one that describes their shapes and orientations. According to the theory, the computation at each stage combines the input from the previous stage with assumptions that are supposedly built into the visual system.