Nearly two thousand years ago a physician called Galen of Pergamon suggested that much of the variation in human behavior could be explained by an individual's temperament. Since that time, ideas about inborn dispositions have fallen in and out of favor. Based on fifteen years of research, Galen's Prophecy now provides fresh insights into these complex questions, offering startling new evidence to support Galen's ancient classification of melancholic and sanguine adults. Two of the most obvious personality traits in children, as well as adults, are a cautious compared with a spontaneous approach to new people and situations. About 20 percent of healthy infants born to loving families come into the world with a physiology that renders them easily aroused by new experiences and, when aroused, to become distressed. A majority of these high-reactive infants become fearful, cautious children. A larger group, about 40 percent of infants, are born with a different physiology that leads them to be more difficult to arouse, but when excited they babble and smile rather than cry. Most of these low-reactive infants become sociable, spontaneous, relatively fearless children. . Galen's Prophecy suggests that each of us inherits a physiology that can affect our moods, leaving some adults dour and tense and others content and relaxed. Integrating evidence and ideas from biology, philosophy, and psychology, Jerome Kagan examines the implications of the idea of temperament for aggressive behavior, conscience, psychopathology, and the degree to which each of us can be expected to control our deepest emotions.