A psychophysiological theory of ORs is presented within the framework of methodological behaviorism. Contrary to O'Gorman there is extensive evidence supporting the distinction between voluntary and involuntary ORs. There is also good evidence indicating that the operation of sets predetermine the occurrence of ORs. A theoretical physiological basis is available to account for the phenomena of set: dominant foci. The latter conception also provides a physiological basis for stimulus significance and permits an integration of diverse phenomena involving ORs, including classical and verbal conditioning.