The foregoing theory of visual proprioception says nothing about volition. It applies as well to passive movements as to active movements. Proprioception is taken to be the awareness of the self that accompanies the perception of the environment. The perception of a goal, its affordance, controls locomotion in one way whereas the visual proprioception of the optical outflow controls locomotion in an entirely different way. A more adequate theory of the steering, guiding, or controlling of behavior can be based on the notion of perceptual systems with built-in proprioceptive functions, and the notion of a general orienting system. The classes of movement so far listed would fall under the kinds of behavior called locomotion, perceptual exploration, and manipulation. The heart of the foregoing classification, however incomplete, is the substitution of postures and movements-between-postures for responses to stimuli.