S-R theorists formulate psychological laws in terms of the relationship between external events and observed behavior. The state of the organism delimits the applicability of the law. Dynamic theorists formulate psychological laws in terms of the relationship between the initial state of the organism Oa and its subsequent state On. The significance of the stimulus is determined by .Oa and the principle of equifinality implies the equivalence of a wide range of behaviors by which On may be reached. It is maintained that regardless of the methodological difficulties inherent in a theory which is based on variables that cannot be observed directly, dynamic theory is testable in principle and in practice. It is suggested that recent neurophysiological theory is incompatible with an S-R approach.