Light falls on our retinae, where neural-circuitry creates the retinal image that is the starting point of visual perception. However, how we "see" or perceive the world is not strictly correlated with our retinal image. The constant, stable environmental structure that we perceive must be derived from but is only partially representative of, a sensory input stream that is noisy and discontinuous. Visual perception, it seems, is a process designed to represent the "permanent possibilities of sensation" (James Mill, 1865). In representing these 'permanent possibilities,' our visual system both subtracts from and adds to our retinal information in order to make salient those aspects of the world that are critical for our purposes and needs. At the same time, our visual system must faithfully represent the true metric of the outside world in our perceptions.