The fundamental problem is the physiology of vision is to understand how perceptual experiences are represented by neural activity. The many contributions that Professor William (Bill) Levick has made in this general field have all been distinguished by their originality, technical ingenuity, experimental precision and meticulous attention to detail. In particular he has provided a formal quantitative theory for sensory neural thresholds, has given a definitive account of the ultimate sensitivity of retinal ganglion cells, has demonstrated that particular classes of neurons encode sophisticated aspects of the visual scene and, in a unique series of experiments, has mapped the individual neurons at one level which connect directly to particular individual neurons at the next level in the brain.