Popper’s initial interest was psychology, not philosophy (Jacobs 1989). His shift from psychology to philosophy occurred around 1928, which was manifested in his doctoral dissertation, ‘On the Problem of Method in the Psychology of Thinking’. Popper’s own writings have mostly ignored this phase. Studies of Popper on psychology have only begun to appear in recent years. Moreover, historians and philosophers have their own version of Popper’s early development (which is related to psychology). We, psychologists, philosophers and historians of psychology should highlight Popper’s psychological phase and can make use of the claims that Popper’s philosophical breakthrough originates from his sharply separating logic and psychology. Careful examinations of Popper’s views are thus important for Psychology of science, deserves thorough reconsideration as psychology can be taken as the trajectory of his subsequent philosophical thought.