The ancient Greek physicians conceptualized man and his diseases in two ways. At the Hippocratic school of Kos the diseased individual was investigated by study of his biography, while at the school of Knidos, disease (the intruder from the outside) was investigated by consideration of pathology. Meerloo asks that we avoid this dichotomy and use rather a plural approach to the understanding, especially the psychological understanding, of man. He surveys the concept of ambivalence, 'the hierarchy of contrasting functions acting on various levels of awareness'; it has analogues in the energic concepts of physics, in biological systems of adaptation, and in psychological and psychoanalytic formulations regarding the psychosexual crises and their contributions to ambivalence and social phenomena. A function of the maturing ego is the ability to integrate the polarities of human existence,—life and death, male and female, pleasure and unpleasure, good and evil, love and hate, and man's inner and outer worlds.