Not only did the man of the Middle Ages see in the regular movements of the celestial spheres the instruments by which God granted him physical existence, but in the various aspects of heavenly phenomena he saw the governing principles of his moral life. The arrangement of the heavenly bodies with regard to one another at various times was supposed to exert undoubted power over the course of terrestrial events. Each planet was thought to have special attributes and a special influence over men's lives. Venus was the planet of love, Mars, of war and hostility, the sun, of power and honor, and so forth. Each was mysteriously connected with a certain color, with a metal, too, the alchemists said, and each had special power over some organ of the human body. The planet's influence was believed to vary greatly according to its position in the heavens, so that to determine a man's destiny accurately it was necessary to consider the aspect of the whole heavens, especially at the moment of his birth, but also at other times. This was called "casting the horoscope" and was regarded as of great importance in enabling a man to guard against threatening perils or bad tendencies, and to make the best use of favorable opportunities. It is not astonishing, then, that the great monuments of literature in the mediaeval period and even much later are filled with astronomical and astrological allusions; for these are but reflections of vital human interests of the times. The greatest poetical work of the Middle Ages, Dante's _Divina Commedia_, is rich in astronomical lore, and its dramatic action is projected against a cosmographical background reflecting the view of Dante's contemporaries as to the structure of the world. Milton, writing in the seventeenth century, bases the cosmology of his _Paradise Lost_ in the main on the Ptolemaic system, but makes Adam and the archangel Raphael discuss the relative merits of this system and the heliocentric view of the universe. The latter had been brought forth by Copernicus a century earlier, but even in Milton's day had not yet succeeded in supplanting the old geocentric cosmology.