Wilhelm Maximilian Wundt, a German physiologist and psychologist is generally known as the founder of experimental, cognitive, and social psychology. His experimental introspection was in contrast to what had been called psychology until then, a branch of philosophy where people introspected themselves. The methods Wundt used are still used in modern psychophysical work, where reactions to presentations of external stimuli are measured. His method then was called introspection, which today would be called observation.

 During the 1890s, an Austrian physician Sigmund Freud, who was trained as a neurologist and had no formal training in experimental psychology, had developed a method of psychotherapy known as psychoanalysis. Freud's understanding of the mind was largely based on interpretive methods and introspection, and was focused in particular on resolving mental distress and psychopathology. Freud's theories became well-known, largely because they tackled subjects such as sexuality and repression as general aspects of psychological development.

Founded by psychologists such as John B. Watson and Edward Thorndike and B. F. Skinner, behaviorism later gained popularity through the mid-20th century as a guiding psychological theory. Animals, rather than humans, were often studied. Behaviorists argued that psychology should be a science of behavior, rather than the mind, and rejected the idea that internal mental states such as beliefs, desires, or goals could be studied scientifically.New Paragraph