 Let us imagine an active CNS sending out orders, or “commands” … to the effectors and receiving signals from its sensory organs. Signals that predictably come when nothing occurs in the environment are necessarily a result of its own activity, i.e., are reafferences. All signals that come when no commands are given are exafferences and signify changes in the environment or in the state of the organism caused by external forces. … The difference between that which is to be expected as the result of a command and the totality of what is reported by the sensory organs is the proportion of exafference…. It is only this difference to which there are compensatory reflexes; only this difference determines, for example during a moving glance at movable objects, the actually perceived direction of visual objects. This, then, is the solution that we propose, which we have termed the “reafference principle”: distinction of reafference and exafference by a comparison of the total afference with the system’s state—the “command”. (Mittelstädt 1971; translated by Bridgeman et al. 1994: 251).