Wundt initiated the first systematic psychological research programme. This achievement occurred at the same time as his elaboration of a philosophy of science which was anti-inductivist and stressed the priority of explanatory motives. Specifically psychological explanations depended on concepts of psychological causality as manifested in apperceptive or volitional processes. The major differences between the Wundtian and other models of psychological experimentation can be understood in the light of this general approach. Thus experimenters and subjects had to be enlightened collaborators and the role of introspection was more significant in an explanatory than in a purely observational context. Wundt's special requirements for the psychological experiment led him to reject other early models as exemplified by the hypnotic experiment in which the experimenter-subject relationship was closer to what was to become the norm in the twentieth century.