Psychology is a fairly young science with roots in many disciplines, ranging from philosophy to physiology. The founder of the first psychology laboratory in 1879 at Germany's University in Leipzig was Wilhelm Wundt, both a physiologist and philosopher. A Russian physiologist who pioneered the study of learning was Ivan Pavlov. Sigmund Freud, an Austrian physician, was a renowned personality theorist. The most influential observer of children was a Swiss biologist, Jean Piaget. William James an American philosopher published the widely used 'Principles of Psychology in 1890.
It appears from this list of psychology pioneers that psychology has its origins not only in many disciplines but also in many countries.
Over a century ago, psychology was defined as the science of mental life. William Wundt focused on the inner experiences of consciousness - feelings, sensations and thoughts. The examination of one's own emotional states and mental processes was the basic research tool at that time.
From about 1920 to 1960, American psychologists, led by John Watson redefined psychology as the science of behavior. You cannot observe a sensation, feeling or thought but you can observe how people's outer behavior are affected by external stimuli.
Since the 1960s, psychology has recaptured its initial interest in conscious and unconscious mental processes. Many psychologists now study how our minds process and retain information.
To encompass psychology's concern with both outer behavior and inner thoughts and feeling, psychology has been defined as the science of behavior and mental processes.
Thus, psychology is a young science but an old subject.