There is no phenomenon in contemporary psychology more striking and puzzling than its theoretical pluralism. Not only is there a plethora of quite different and apparently irreconcilable theories in different subdomains of psychology—physiological psychology, learning, cognition, motivation, personality, developmental psychology, social psychology, psychotherapy—but even within a single field such as psychotherapy, for example, there is a steady increase in the number of new theories. When I was a graduate student in psychology there were already 36 systems of psychotherapy and by now the number may well have doubled. I have lost count.