One of the few truly general ideas that we have in neurophysiology is that organisms try to reduce deviations from their normal states. This idea, which has a long and interesting history, is now widely known as the ‘cybernetic principle’, or, more descriptively perhaps, the ‘negative feedback principle'. It holds that an adaptive system can maintain a stable state in a fluctuating environment if it can sense deviations from that state and initiate actions to reduce them. Probably the most familiar physiological examples are the homeostatic mechanisms that maintain the constancy of the internal environment, and the system of postural reflexes that tend to restore an animal’s original position with respect to gravity whenever it has changed. Although the cybernetic principle applies more directly to neurophysiological systems, it has also been adopted gratefully by many psychologists. What appears at a physiological level as an effort to maintain homeostatic equilibrium emerges at a behavioural level as a system of drives that motivate vital behaviour. And what appears at a physiological level as feedback of information about deviations from an expected result emerges at a psychological level as a complex perceptual system for the guidance of coordinated action. Thus, psychological theories of both motivation and perception have been strongly affected by this negative feedback principle borrowed from electrical engineering.