The end of the 19th century marks the start of psychology as a scientific enterprise. The year 1879 is commonly seen as the start of psychology as an independent field of study, because in that year Wilhelm Wundt founded the first laboratory dedicated exclusively to psychological research (in Leipzig). Other important early contributors to the field include Hermann Ebbinghaus (a pioneer in studies on memory), Ivan Pavlov (who discovered the learning process of classical conditioning), and Sigmund Freud. Freud's influence has been enormous, though more as cultural icon than a force in (scientific) psychology. Freud's basic theories postulated the existence in humans of various unconscious and instinctive "drives", and that the "self" existed as a perpetual battle between the desires and demands of the internal id, ego, and superego.
The 20th century saw a rejection of Freud's theories as being too unscientific, and a reaction against Edward Titchener's abstract approach to the mind. This led to the formulation of behaviorism by John B. Watson, which was popularized by B.F. Skinner. Behaviorism proposed epistemologically limiting psychological study to overt behavior, since that could be quantified and easily measured. Scientific knowledge of the "mind" was considered too metaphysical, hence impossible to achieve. The final decades of the 20th century have seen the rise of a new interdisciplinary approach to studying human psychology, known collectively as cognitive science. Cognitive science again considers the "mind" as a subject for investigation, using the tools of evolutionary psychology, linguistics, computer science, philosophy, and neurobiology. This new form of investigation has proposed that a wide understanding of the human mind is possible, and that such an understanding may be applied to other research domains, such as artificial intelligence.
The Ebers papyrus (ca 1550 BC) contains a short description of clinical depression. Though full of incantations and foul applications meant to turn away disease-causing demons and other superstition, it also evidences a long tradition of empirical practice and observation.
From its inception, a great deal of work in philosophy, especially in the field of epistemology, dealt with the nature of the mind, its processes, and its contents, though usually in a theoretical (non-empirical) fashion.
The German scholastic philosopher Rudolph Goclenius was the first to use the term psychology in 1590.
The root of the word psychology (psyche) means "soul" or "spirit" in Greek, and psychology was sometimes considered a study of the soul (in a religious sense of this term), though its emergence as a medical discipline can be seen in Thomas Willis' reference to psychology (the "Doctrine of the Soul") in terms of brain function, as part of his 1672 anatomical treatise "De Anima Brutorum" ("Two Discourses on the Souls of Brutes").
Until about the end of the 19th century, psychology was regarded as a branch of philosophy.
In 1879 Wilhelm Wundt founded a laboratory at the University of Leipzig in Germany, specifically to focus on general and basic questions concerning behaviour and mental states. Then in 1890, William James published the book Principles of Psychology which laid many of the foundations for the sorts of questions that psychologists would focus on for years to come. James was the first professor of Psychology at Harvard University. Crucially, the approach of Wundt and James did not involve metaphysical or religious explanations of human thought and behaviour, freeing it from the realms of philosophy and theology, founding the modern science of psychology.
Of great influence to the development of this new scientific field was the establishment of the American Psychological Association in 1892.