Those who think they know all about the subject because they have read Macrobius and a few other authors are found wanting in a crisis,--a passage meant doubtless as a hit at those who base their knowledge of astronomy simply upon Latin authors. Pedro also alludes to those who have been accustomed to regard themselves as teachers of astronomy and now hate to turn pupils again. [Sidenote: Experimental method.] The contrast which Pedro draws, however, is not so much between Latin and Arabic writings as it is between dependence upon a few past authorities and adoption of the experimental method. He argues that the principles of astronomy were discovered in the first place only through experimentation, and that today no one can understand the art fundamentally without actual observation and experience. He believes that astrology as well as astronomy is proved by experience. “It has been proved therefore by experimental argument that we can truly affirm that the sun and moon and other planets exert their influences in earthly affairs.”[170] Or, as he says in another passage, “And indeed many other innumerable things happen on earth in accordance with the courses of the stars, and pass unnoticed by the senses of most men, but are discovered and understood by the subtle acumen of learned men who are skilled in this art.”[171] Pedro’s letter further includes some astrological medicine, interesting in connection with the statement in another manuscript that he was the physician of Henry I of England. In this context, too, he shows familiarity with the translations from the Arabic of Constantinus Africanus.[172] [Sidenote: Magic and scepticism in the _Disciplina clericalis_.]