Few scientific data remain valid if isolated from the philosophical stream that bore them, from the social setting where they were elaborated, and from the instrumentation that characterized the research of the period in which they originated. Pavlov's contribution, the introduction of the method of conditioning and the recognition of the implications of the conditional reflex, is among those which do. A critical evaluation of Pavlov's work with all its ramifications in a present-day scientific frame of reference has to be based on a comprehensive understanding of closely interrelated scientific disciplines. Conditioning is a behavioral method, and Pavlov assumed that there exists in the central nervous system a step-by-step representation of the various patterns of observable human behavior. Experiments by other investigators applying other methods contradicted Pavlov's inferences; for instance, strych-ninization revealed an entirely different kind of cortical organization from that which Pavlov had assumed.