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The psychology of thinking, animal psychology, and the young Karl Popper.

In the 1920s, Karl Popper wrote two large manuscripts on psychology that he never published. In his autobiography, Unended Quest, he attempts to reduce the importance of his work in psychology as much as possible, and in his philosophical work he is an antipsychologist. However, in this article, it is argued that Popper's early psychology has been pivotally important for the development of his philosophy. In particular, it is shown that Popper's views on psychology underwent a radical shift, one that paved the way for his characteristic deductive stance in philosophy. Popper's views shifted from an inductive and associationistic psychology toward a noninductive psychology of problem solving. Tracing the historical background of Popper's early work reveals how he integrated various parts of the psychology of Karl Groos into his analysis of the childish phenomenon of dogmatic thinking and how he shortly after appropriated various elements of the animal psychology of Hans Volkelt and Herbert Jennings in his biological approach to (dogmatic) thinking. In the monumental works of Otto Selz, however, Popper finally found the roots of a noninductive and biological approach to the growth of individual and scientific knowledge.