A pattern of luminances equivalent to that of a traditional simultaneous lightness display (two equal gray squares, one on a white background and the other on an adjacent black background) was presented to observers under two conditions, and matches were obtained for both perceived reflectance and perceived illumination level of the squares and their backgrounds. In one condition, the edge dividing the two backgrounds was made to appear as the boundary between a white and a black surface, as in the traditional pattern. The squares then were perceived as almost the same shade of middle gray. In the other condition, a context was supplied that made the edge between the backgrounds appear as the boundary between two illumination levels, causing one square to appear black and the other white. These results were interpreted as a problem for local ratio theories, local edge theories, and lateral inhibition explanations of lightness constancy, but as support for the concepts of edge classification, edge integration, and the retinal image as a dual image.Theories that appear different or even opposite may nonetheless share a common starting assumption, and the preoccupation with such apparent differences may serve to obscure flaws in the common assumption. Believing this to be true for theories of lightness perception, we have chosen an alternative, and somewhat radical, starting point for the experiments we report here. This approach will be more easily grasped if we begin by reviewing the central role of luminance information in current theories of lightness perception.The Photometer Metaphor: Absolute Lumiuauce Levels as the Basic Iuput Implicit in most theory and research on the perception of surface lightness is a conception we would call the photometer metaphor. This term refers to the assumption that, fundamentally, the visual system measures the intensity of light reflected by each point in a visual scene. Helmholtz and Hering, although their views were usually in opposition, both assumed that absolute intensity of reflected light (luminance) was available as an input to the visual system. Of course, neither their theories of lightness perception nor the more modem theories are