Metaphysical behaviorism of the sort espoused by John B. Watson (1878–1958) and his followers makes a much stronger claim. It denies the existence of mind and consciousness and proposes that all mentalistic concepts be properly defined (or redefined) in terms of observable behavior. Watson maintained that behavior can be explained entirely in terms of stimulus and response, without the intervention of mental or conscious events and activities. For Watson, all behavior is environmentally derived and cannot be explained by appeals to heredity, instincts, the unconscious, human nature, or internal predispositions. Some behaviorists recognize two different kinds of observable behavior: external behavior, which is sometimes characterized as overt, external, or molar (pertaining to the whole); and internal behavior, which is alternatively called covert, implicit, deep, or central behavior. If thinking is defined as “talking” or “speaking,” an account must be given of what transpires when people are thinking silently “to themselves.” The wife of a philosopher once complainedthat she could never tell whether he was working or loafing. Many psychological processes and activities seem, at times, to involve no external behavior. Behaviorists may either deny the reality of private events or affirm that they involve internal behaviors or processes. Thus, thinking becomes “motion in the head,” as Thomas Hobbes (1588–1679) put it, or “sub-vocal speech,” as Watson suggested.Behaviorism is usually associated with some form of metaphysical materialism, of which there are many varieties (Foss). When internal behavior is identified with neurophysiological activity, behaviorism becomes centralstate materialism, or neuromaterialsim, according to which the reality of mental states and processes is identical with that of physical states and processes in the brain and central nervous system. This theory identifies mental processes with electrical and chemical processes within the central nervous system (“motion in the head”)