Psychology as the behaviorist views it is a purely objective experimental branch of natural science. Its theoretical goal is the prediction and control of behavior. Behaviorism's philosophical roots go back to ancient Greek atomic theory, the first complete mechanistic and materialistic psychology. American behaviorism was rooted in familiar philosophical and scientific soil dating back to the early work of philosophical materialists. Behaviorism was appealing because it offered a direct and unambiguous message. In an overview of the behaviorist's career, G. Bergmann declared that John B. Watson was second only to S. Freud as "the most important figure in the history of psychological thought during the first half of the century". The neobehaviorists were largely in agreement with operationism and with a related movement in philosophy called logical positivism. Neobehaviorism's reign in the 1930s, 1940s, and 1950s produced an unprecedented outpouring of research. The neobehaviorists who followed in the tradition of Watson also worked on practical problems.