The main function of a scientific theory is to help us to describe and explain how the world works. To form a scientific theory, scientists start with a set of assumptions, then use logic and deductive reasoning to come up with hypotheses that they can test. Psychologists use scientific theories to attempt to explain human thought, emotion, and behavior. According to your textbook, Theories of Personality, there are six criteria of a useful scientific theory. A useful theory: generates research, is falsifiable, organizes data, guides action, is internally consistent, and is parsimonious (simple).