The sensitivity of the eye to two kinds of continuous transformation of a shadow-pattern.

put a psychological question which has not yet been answered by psychologists: How does the eye distinguish between the change of position of a thing and the change of state of a thing? By "position, " he meant the distance and orientation of an object in the environment, which can change when either the observer or the object moves. By "state, " he meant the identity or rigidity of an object in the environment, which can change when the object grows or changes shape (even to the extent of becoming a different