William T. Powers (August 29, 1926 - May 24, 2013) was an independent scientist who developed perceptual control theory (PCT). PCT posits, and demonstrates, that living things do not control their behavior, but rather their behavioral outputs are the variable means by which they control their sensory inputs (perceptions). Engineering control theory differs from PCT by specifying a separate controller external to the system, whereas in living systems the reference variable for each control loop in a control hierarchy is set from within the system. Powers and his students and colleagues in diverse fields have developed many demonstrations of negative feedback control, 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).
Powers, William T. (2008). Living Control Systems III: The fact of control. [Mathematical appendix by Dr. Richard Kennaway. Includes computer programs for the reader to demonstrate and experimentally test the theory.] New Canaan, CT: Benchmark Publications. ISBN 978-0-9647121-8-8.