Psychology is generally defined as the science of behavior and mental processes and the application of the resulting findings to the solution of problems. The word thus simultaneously refers to a science (involving the study of the behavior of humans and animals) and to various interventions (treatments and therapies) in the mental processes and behavioral patterns of people. Psychology differs from sociology, anthropology, economics, and political science, in part, by studying the behavior of individuals (alone or in groups) rather than the behavior of the groups or aggregates themselves. While psychological questions were asked in antiquity (c.f., Aristotle's De Memoria et Reminiscentia or "On Memory and Recollection"), psychology emerged as a separate discipline only recently. The first person to call himself a "psychologist", Wilhelm Wundt, opened the first psychological laboratory in 1879.
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").
Experimental psychology, as introduced by Wilhelm Wundt in 1879 at Leipzig University in Germany, eliminated religious implications from psychology entirely. Today, experimental psychology focuses on observable behavior and the evidence it gives about mental processes. It therefore has little specific to say about such notions as an immaterial, immortal soul. Modern psychology is often called the scientific study of behavior, though (as in cognitive psychology) its purported object is often not behavior but various mental events. There are also now many psychological approaches that attempt to take spiritual and religious issues seriously.
Until about the end of the 19th Century, psychology was regarded as a branch of philosophy. With the work of Wundt and of his contemporary experimental psychologist William James (who, himself, questioned the veracity of materialistic psychology in his later work), the field of psychology was slowly but steadily established as a science independent of philosophy. Of course, like all sciences that have broken off from philosophy, purely philosophical questions about the mind are still studied by philosophers; the name of the philosophical subdiscipline which studies those questions is philosophy of mind. Few universities, journals, or researchers today treat psychology as a branch of philosophy, but there is much disagreement as to whether it should be considered an experimental science. Some academic psychologists are still of the opinion that psychological understanding can only progress through rigorously controlled laboratory experiments, but most now accept that less carefully controlled quantitative methods (such as survey research) as well as qualitative research are equally valuable.
Psychologists work in co-operation (and sometimes in competition) with psychiatrists (who are medical doctors who specialize in mental health issues), social workers (most of whom are qualified in various forms of psychological intervention), psychiatric nurses and 'lay' counselors. Services similar to those provided by psychologists are also often provided by traditional healers and religious counselors. Fields such as neuroscience, political science, media studies and gender studies have also come to be seen as closely related to psychology.
Both psychology and its related discipline psychiatry (whose practitioners are medical doctors with a specialty in mental health) are criticized by a vocal and well-credentialed (if small) minority in medical and academic circles. These critics call them pseudo-sciences, arguing that their theories, diagnoses and treatments don't hold up under the rigor of the scientific method and that they are not reproducible; others question the appropriateness of applying the scientific method to the study of the human mind and human behavior. A related view is promulgated by some philosophers under the label eliminative materialism. These challenges to the discipline are, in large part, legitimate and needed, especially when one considers the discipline's growing influence in Western culture and how easy it can be to construct psychological models that are considered untestable (e.g., Freud's model of the psyche). These concerns seek not to subvert psychology but to strengthen it by the same rigorous inquiry present in other sciences.
Whether psychology can follow the model of the natural sciences, however, is still yet to be fully tested. While some fields of psychology, such as reinforcement theory, have clearly produced reproducible results using scientific analysis, the scientific status of other fields remains doubtful. To some, psychology is a 'critical' science rather than an empirical one, testing things not of the natural world but mere structures and concepts based on our own social constructions. Whether the mind can be overall looked at as if nothing more than chemicals remains a contentious issue, and the very methodologies, approaches, meanings and possible results of psychology are still being discovered.
Topics in Psychology
Major Nineteenth and Twentieth Century Schools of Thought
Divisions and Approaches in Psychology (these might be overlapping, of course)
Some related disciplines:
For a fuller list of topics, please see the list of psychological topics.