Psychology is the science of behavior and the mind, including both conscious and unconscious phenomena. Ancient civilizations like Egypt, Greece, India, and Persia engaged in early philosophical study of psychology. In the 4th century BC, Greek physician Hippocrates theorized that mental disorders had physical rather than supernatural causes. Health psychology studies psychological and behavioral processes related to health, illness, and healthcare. Personality refers to enduring psychological qualities that influence patterns of behavior and can be understood through trait, psychodynamic, humanistic, social-learning, cognitive, and self theories. Traits describe consistent qualities or attributes that predispose individuals to behave in certain ways across situations.