Perception is the having and achieving of knowledge about the world, and visual perception is the most exact kind of perception. The sequence of fleeting retinal images is scarcely detectable in perception; what emerges is a phenomenal scene together with its interesting features. There are different degrees of mediation for different kinds of perception, all the way from the direct impression to the roundabout inference. The surface has been treated or processed or acted upon in such a way that the light causes a perception of something other than the surface itself. The makers of pictures, at least those with commercial ambitions, have generally wanted to remedy the defects in the lifelike quality of pictorial perception. The ordinary picture is a selection from the total scene—a choice of a certain angular sector which the viewer is permitted to observe. The static picture can represent, it is true, a critical moment in time.