Greek philosophers of the sixth century b.c. were interested in the senses as subjects of investigation and recognized the brain as the seat of all mental activity. Aristotle, a Greek philosopher of the fourth century b.c., wrote the first psychological treatise, in which he described five senses and discussed memory and dreams. However, he thought that the heart, rather than the brain, was the center of mentality.

During the Middle Ages, little progress was made in the investigation of behavior. With the Renaissance, interest in psychological phenomena reawakened. Leonardo da Vinci, an Italian artist and engineer, made some of the first valuable studies of visual perception. Other work in perception was done in the 17th century by Johannes Kepler, a German scientist, and Isaac Newton, an English scientist.

Thomas Hobbes, a 17th-century English philosopher, advanced the understanding of psychology as a natural science. He stressed the part played by the senses in determining mental activity. Hobbes also discovered that ideas could be associated in the mind without conscious direction. His discovery and theories were developed by others, such as David Hume, a Scottish philosopher, into the school of association psychology.

During the 17th, 18th, and early 19th centuries, conflicting theories of philosophy were offered by leading European thinkers. Each, however, contributed to the knowledge of mental processes and behavior.

Psychology as a modern science began in the 19th century with the work of four German scientists: Ernst H. Weber, physiologist and anatomist; Gustav T. Fechner, physicist and psychologist; Hermann L. F. von Helmholtz, physiologist and physicist; and Wilhelm Wundt, physiologist and psychologist.

Weber measured the relation between sensory stimulation and sensation. His work was elaborated by Fechner. Together they founded the branch of science known as psychophysics, which deals with phenomena common to both psychology and physics. Von Helmholtz made important contributions to theories of visual and auditory perception. Wundt is generally considered to be the founder of experimental psychology. He established the first psychological laboratory at the University of Leipzig in 1879.

The methods of experimental psychology were employed to develop many theories, including those concerning learning, memory, intelligence, and conditioned reflexes. For example, Hermann Ebbinghaus, a German psychologist, demonstrated in 1885 that learning and memory could be investigated by scientific methods and be measured quantitatively. He helped to bring the study of learning and memory out of the realm of philosophy into that of psychology. Alfred Binet and Thodore Simon, French psychologists, devised the first successful standardized intelligence test in 1905.

Among other important contributions to psychological theory and research were those of workers in the field of biological evolution. With the publication of Darwin's Origin of Species (1859) and his Expressions of Emotions in Man and Animal (1873), new emphasis was placed on comparative psychology. Darwin and other researchers showed that much human behavior that appears meaningless is based on reactions that are necessary to the survival of less highly developed species. For example, a man may bare his teeth when he is angry. In other mammals, such as a wolf or a baboon, showing the teeth in this fashion is a warning that the animal will bite.

Another important study was made in the field by those interested in more subjective methods of inquiry. In 1895 Sigmund Freud and Josef Breuer, Austrian physicians, published Studies in Hysteria, which marked the beginning of the psychoanalytic school. The theory of psychoanalysis was further expounded in Freud's Interpretation of Dreams (1900). Among others who worked with Freud and made valuable contributions to psychoanalytic theory were Alfred Adler and Carl Jung, both of whom broke away to found psychological schools of their own.

Influential schools of psychology established in the 20th century include behaviorism, introduced by John B. Watson, a United States psychologist. According to this theory, psychology should be concerned only with objective, observable behavior.

Gestaltism is another theory that has had impact on 20th-century psychology. According to this theory, the whole experience is greater than the sum of its component parts. Kurt Koffka and Wolfgang Khler, German-American psychologists, and Max Wertheimer, a German psychologist, were influential in developing the gestalt school.

The trend in 20th-century psychology is toward an experimental school that includes many of Wundt's theories, as well as those of behaviorism and gestaltism. Clinical psychologists use Freudian methods, as well as less subjective ones derived from the experimental schools, in diagnosis and treatment of the mentally disturbed.

is the most influential psychological, organization in the United States. The APA, founded in 1892, has some 118,000 members. Headquarters are in Washington, D.C.