Perceptual control theory in the context of Negative feedback

Equifinality is the principle that in open systems a given end state can be reached by many potential means. The term and concept is due to the German Hans Driesch, the developmental biologist, later applied by the Austrian Ludwig von Bertalanffy, the founder of general systems theory, and by William T. Powers, the founder of perceptual control theory. Driesch and von Bertalanffy prefer this term, in contrast to "goal", in describing complex systems' similar or convergent behavior. Powers simply emphasised the flexibility of response, since it emphasizes that the same end state may be achieved via many different paths or trajectories.

In closed systems, a direct cause-and-effect relationship exists between the initial condition and the final state of the system: When a computer's 'on' switch is pushed, the system powers up. Open systems (such as biological and social systems), however, operate quite differently. The idea of equifinality suggests that similar results may be achieved with different initial conditions and in many different ways. This phenomenon has also been referred to as isotelesis (from Greek ἴσος isos "equal" and τέλεσις telesis: "the intelligent direction of effort toward the achievement of an end") when in games involving superrationality.

William T. Powers (August 29, 1926 – May 24, 2013) was a medical physicist and an independent scholar of experimental and theoretical psychology who developed the perceptual control theory (PCT) model of behavior as the control of perception. He was the son of the well-known cement scientist and economist Treval Clifford Powers.

PCT demonstrates that rather than controlling their behavioral outputs, living things control their perceptual inputs, and explains how they vary their behavior as the means of controlling inputs to their sense organs. Living control systems differ from those specified by Engineering control theory (a thermostat is a simple example), for which the reference value (setpoint) for control is specified outside the system by what is called the controller, whereas in living systems the reference variable for each feedback control loop in a control hierarchy is generated within the system, usually as a function of error output from a higher-level system or systems. Powers and his students and colleagues in diverse fields have developed many demonstrations of autonomous negative feedback control with endogenously generated reference values, and computer models or simulations that replicate observed and measured behavior of living systems (human and animal, individuals and groups of individuals) with a very high degree of fidelity (0.95 or better). Some corresponding control structures have been demonstrated neurophysiologically.