On these constants are superposed minor changes of shape that constitute her behavior and her expressive movements. An object, then, is a persistently and characteristically shaped surface. A blade of grass, a tree, and even a house are objects although they are not detached from the ground. A discrete object is detached and movable, like a ball, a book, an automobile, or a man. We can now bring in occlusion, i.e., the fact of the projection or non-projection of surface at points of observation.
Objects always occlude.
The back of the object and the background are unprojected. But the observer can always walk around the object, or turn it around, and this is presumably the natural way of perceiving an object. The back of the object is always turning into the front of the object. The surface of the earth, on the other hand, does not occlude, and the ground is not an object. The substances of the environment do not hide anything behind them.