This chapter analyzes the stimulus information that is available to an observer, formulate hypotheses about the process of perception, and finally test them with experiments that provide stimulus information. The distinguishing of objective motion and subjective movement is an old puzzle for the theory of perception but the solution may prove to be simpler than has been supposed. Water or glass or swirling mist yields the perception of one thing behind another, but without opaque occlusion. E. J. Gibson, Gibson, N. K. Smith, and H. Flock displayed an array with two sets of interspersed units. The fact that one can drive a car on a rainy night, with an optic array coming from multiple moving reflections and changing blurs on the glass windshield, and nevertheless see the road instead of a chaos bears witness to the mathematical wisdom of the visual system.