Personality is defined as the characteristic sets of behaviors, cognitions, and emotional patterns that evolve from biological and environmental factors. While there is no generally agreed upon definition of personality, most theories focus on motivation and psychological interactions with one's environment. Trait-based personality theories, such as those defined by Raymond Cattell, define personality as the traits that predict a person's behavior. On the other hand, more behaviorally-based approaches define personality through learning and habits. Nevertheless, most theories view personality as relatively stable. The study of the psychology of personality, called personality psychology, attempts to explain the tendencies that underlie differences in behavior. Many approaches have been taken on to study personality, including biological, cognitive, learning and trait-based theories, as well as psychodynamic, and humanistic approaches. Personality psychology is divided among the first theorists, with a few influential theories being posited by Sigmund Freud, Alfred Adler, Gordon Allport, Hans Eysenck, Abraham Maslow, and Carl Rogers.