The overarching obstacle in the evolution of vision, however, was recognized several centuries ago by George Berkeley, who pointed out that the information in images cannot be mapped unambiguously back onto real-world sources. In contemporary terms, information about the size, distance and orientation of objects in space are inevitably conflated in the retinal image. In consequence, the patterns of light in retinal stimuli cannot be related to their generative sources in the world by any logical operation on images as such. Nonetheless, to be successful, visually guided behavior must deal appropriately with the physical sources of light stimuli, a quandary referred to as the "inverse optics problem". As briefly explained here, visual illusions appear to arise primarily from the way the visual system solves this problem.