The document discusses Carl Jung's theory of personality types and psychological types. It explains that Jung believed behaviors are not random and that differences between individuals result from how they prefer to use their mental capacities. Jung proposed that people typically take in information or make decisions, and prefer to be alone or with others. This led Jung to write Psychological Types, which introduced the concepts of personality types. The document then provides an overview of introversion as one of Jung's personality types, describing key characteristics of introverts.