As a basis for understanding the visual system, we need to consider the functions that vision has to perform, which are pre-eminently in the service of activity, and the circumstances in which it normally operates, namely when the head is moving. The fundamental ecological stimulus for vision is not a camera-like time-frozen image but a constantly changing optic array or flow field, the description of which must be in spatio-temporal terms. A mathematical analysis of the optic flow field is presented, revealing the information that it affords for controlling activity - information both about the topography of the environment and about the movement of the organism relative to the environment. Results of human behavioural experiments are also reported. It is suggested that the optic flow field should be the starting point in attempting to discover the physiological workings of the visual system.