Excerpt: "Psychology has been defined in many ways. For much of the twentieth century psychology was defined as the *science of behaviour. Earlier, and again more recently, psychology has also emphasized internal experiences, such as sensations, perceptions, feelings or emotions, motivations and thoughts. Today, most definitions of psychology include both elements. Thus, we will define psychology as the science of behaviour and mental processes."
Bufford, Rodger K., "Psychology" (2006). Faculty Publications - Grad School of Clinical Psychology. 304.