  Why has the influence of behaviorism declined?  The deepest and most complex reason for behaviorism’s decline in influence is its commitment to the thesis that behavior can be explained without reference to non-behavioral and inner mental (cognitive, representational, or interpretative) activity.  Behavior, for Skinner, can be explained just by reference to its “functional” (Skinner’s term) relation to or co-variation with the environment and to the animal’s history of environmental interaction. Neurophysiological and neurobiological conditions, for Skinner, sustain or implement these functional or causal relations.  But they do not serve as ultimate or independent sources or explanations of behavior.  Behavior, Skinner (1953) wrote, cannot be accounted for “while staying wholly inside [an animal]; eventually we must turn to forces operating upon the organism from without.” “Unless there is a weak spot in our causal chain so that the second [neurological] link is not lawfully determined by the first [environmental stimuli], or the third [behavior] by the second, the first and third links must be lawfully related.” (p. 35) “Valid information about the second link may throw light on this relationship but can in no way alter it.” (ibid.)  It is “external variables of which behavior is a function.” (ibid.) 