As a consequence of this application of astrological knowledge to medicine, operations were performed only on certain favorable days or under favorable conjunctions of planets. An ailment that occurred at an unfavorable time, because of an unpropitious state of the heavens, would not be relieved until the motions of the stars brought a more benign conjunction. Observations seemed clearly to indicate that the stars actually had such influences. Even Hippocrates, though he insisted that "the medical art requires no basis of vain presumption, such as the existence of distant and doubtful factors, the {39} discussion of which, if it should be attempted, necessitates a hypothetic science of supra-terrestrial of subterrestrial belief," could not entirely get away from astrology. In his treatise on "Air, Water and Locality" he writes: "Attention must be paid to the rise of the stars, especially to that of Sirus as well as the rise of Arcturus, and after these to the setting of the Pleiades, for most diseases in which crises occur develop during these periods." In the second chapter he writes: "If anyone would be of the opinion that these questions belong solely in the realm of astrology, he will soon change his opinion as he learns that astrology is not of slight, but of very essential importance in medical art." (Personally I doubt the Hippocratean authorship of these passages, but they are surely very old.) The influence of the suggestions derived from astrology on human patients continued until almost the nineteenth century. There were many protests, especially from the Doctors of the Church, that the applications of astrology to medicine were false, but the practice continued. Both Kepler and Galileo drew horoscopes for patrons, and while Kepler doubted their value, he felt that in making them he was justified by custom. Galileo drew up the horoscope of the Grand Duke of Tuscany during an illness, and declared that the stars foretold a long life, but the Duke died two weeks later. But incidents of this kind did not disturb either popular faith or medical confidence in astrology as helpful, in prognosis, at least, if not also in diagnosis. Even so late as 1766 Mesmer was graduated at the University of Vienna, when it was doing the best medical work in Europe, with a thesis on "The Influence of the Stars on Human Constitutions."