This document provides an overview of theories of personality, including Carl Jung's concepts of introversion and extroversion and Hans Eysenck's theory of the biological basis of introversion and extroversion. Eysenck believed that differences in central nervous system functioning, specifically the reticular activating system, underlie personality types, with high ARAS arousal predisposing individuals to introversion and low ARAS arousal to extroversion. The document also discusses research findings related to biological differences between introverts and extroverts and Eysenck's view that biological predispositions interact with environmental factors to shape personality.