Color isn"t physical properly related to physical property. Most of light we see is reflected light. More light absorbed, darker surface will appear. Lights of 450 and 625 nm each elicit same response from photoreceptor. No way of distinguishing between 2 lights: when seeing color, output of single photoreceptor is ambiguous, problem of variance: fact that infinite set of different wavelength-intensity combinations can elicit exactly same response from single type of photoreceptor. One photoreceptor type can"t make color discriminations based on wavelength. Human retina contains 2 photoreceptors rods and cones: rods sensitive to low (scotopic) light levels. Scotopic: light intensities that are bright enough to stimulate rod receptors but too dim to stimulate cone receptors. All rods contain same type of photopigment molecule rhodopsin same sensitivity to wavelength. As consequence, though possible to tell light from dark under scotopic conditions, problem of univariance makes it impossible to discriminate colors color is psychophysical: cone photoreceptors sensitive to higher, daylight levels.