  Computationalists regard the inescapable poverty of stimuli as placing on cognitive systems a need to draw inferences. Just as background knowledge allows you to infer from the footprints in the snow that a deer has passed by, cognitive systems, according to computationalists, rely on sub-conscious background knowledge to infer what the world is like given the partial clues the stimuli offer. The perception of an object’s size, for instance, would, according to the computationalist, be inferred on the basis of the size of the retinal image of the object together with knowledge of the object’s distance from the viewer. Perception of an object’s shape, similarly, is inferred from the shape of the retinal image along with knowledge of the object’s orientation relative to the viewer.