If one is really to perceive an object, one must perceive the ground extending behind the object, as Koffka pointed out. And this is not all, for we must also perceive the back surface of the object. There is a great deal of current confusion about occlusion, and about the philosophical implications of the perception of hidden things, and about its development in the child. It would be useful therefore to describe it in the terms of ecological optics. No description in terms of physical optics exists.
Occlusion occurs only to the extent that surfaces are opaque.
That is to say some things are unhidden and others hidden, some visible and other invisible, some revealed and some concealed. All occlusion is caused by what I will call an occluding edge . An occluding edge entails two surfaces, one that occludes and one that is occluded. Hence the perception of an occluding edge involves the perception of two such surfaces, one superposed on another. An occluding edge implies depth at the edge, that is, a step or increase of distance on the non-occluding side.