Harry K. Wells in his book stage by stage traces the exciting history of the discovery and study of the conditional reflex by I.P. Pavlov - from the first observations of mentally determined saliva secretion to the study of the physiological foundations of hypnosis, the first and second signaling systems, neuroses, psychoses and variants of the human temperament. The reader of Wells's book, as it were, along with I.P. Pavlov is present in his laboratories, reflects with him on the mysteries of nature and reveals the greatest secrets of the material substrate of the human spirit.