THE permanence of the influence of Aristotle (384-322 B.C.) justifies Dante's description of him as " the Master of those who know." In every branch of learning, Aristotle based himself on close observation of fact. His pupils made a collection of a hundred and fifty different constitutions to form the basis of a theory of government. Alexander the Great, who had been his pupil, sent him specimens of animals and plants for his studies in biology and botany. In logic and philosophy, in ethics and politics and literature, he both summarised past learning and extended the bounds of human knowledge. His collected works form an encyclopaedia of learning, so imposing that succeeding ages neglected to look beyond them and search out truth for themselves. In the study of medicine, Greek lucidity of thought was revolutionary. Earlier peoples, believing that disease was sent by God, had tried to cure it by a mixture of magic and drugs. The new view was that all symptoms were equally divine, but that all obeyed natural law, which knew no exceptions. It was the doctor's duty to examine symptoms, and to decide on the result of that examination. It seems obvious enough now, but the Greeks pointed it out first.