Astrology, as a science and a system of divination, exerted a profound influence over the mediaeval mind. No court was without its practicing astrologer and the universities all had their professors of astrology. The practice of astrology was an essential part of the physician's profession, and before prescribing for a patient it was thought quite as important to determine the positions of the planets as the nature of the disease.[139] Interesting evidence of this fact is found in the _Prologue to the Canterbury Tales_ where Chaucer speaks of the Doctour's knowledge and use of astrology as if it were his chief excellence as a physician: "In al this world ne was ther noon him lyk To speke of phisik and of surgerye; For he was grounded in astronomye. He kepte his pacient a ful greet del In houres, by his magik naturel. Wel coude he fortunen the ascendent Of his images for his pacient."[140] Yet in spite of the esteem in which astrological divination was held by most people in the Middle Ages, Dante, the greatest exponent of the thought and learning of that period, shows practically no knowledge of the technical and practical side of astrology. When he refers to the specific effects of the planets it is only to those most familiarly known, and he nowhere uses such technical terms as "houses" or "aspects" of planets. But Dante, like the great philosophers of the earlier periods, was undoubtedly influenced by the philosophical doctrines of astrology, and a general belief in the influence of the celestial spheres upon human life was deeply rooted in his mind. To him the ceaseless and harmonious movements of the celestial bodies were the manifestations and instruments of God's providence, and were ordained by the First Mover to govern the destinies of the earth and human life.