The oldest and simplest method, then of describing differences in personality was to classify people according to types. Such a system is called a Typology. A famous example of this method was set forth in Greece about the year 400 B. C.. A physician named Hippocrates theorized that there were four fluids, or humors, in the body. Corresponding to each humor, he believed, there existed a definite type of personality. The four humor were blood, yellow bile(胆汁), black bile, and phlegm(黏液). A person in whom all four humor were in perfect balance had a harmonious(和谐的)personality. If a person had too much blood, he was called sanguine, or cheerful and optimistic. Someone with too much yellow bile was choleric, or irritable and easily angered. Too much black bile made a person melancholy, or depressed and pessimistic. An oversupply of phlegm caused a human being to be phlegmatic, or slow and unfeeling. Scientists have long since discarded Hippocrates fluid theory. But the names of the humors, corresponding to these temperaments(气质), have survived and are still useful, to some extent, in describing personality. Other features of people, such as their faces and physiques, have also been used to classify personality. Today, however, personality theories and classifications may also include factors such as heredity, the environment, intelligence, and emotional needs. Psychology, biology, and sociology are involved in these theories. Because of the complexity of human personality, present-day theories are often very different from one another. Psychologists vary in their ideas about what is most important in determining personality. This passage focuses on______.