During the past 150 years, many scientists, fascinated by the achievements of the visual systems of humans and animals, have discovered some of the ways in which vision acquires knowledge of the external world. This volume reviews seven major theoretical approaches to the problem of explaining vision: psychophysics and nature of sensory thresholds; the Gestalt Theory; probabilistic functionalism; neurophysiological theories; empiricism; the theory of direct perception; and the computational approach to vision. The philosophical and historical background to each theory is described, and a final summary chapter assesses the strengths and weaknesses of each theory and attempts to predict future developments.