Introduction to Astronomy

the harvest of such knowledge is often not small, especially in the foreknowledge of events. We see, however, that both physicians and others of that kind are sometimes deceived in their respective crafts. Yet, for this reason, one should neither refuse their help nor despise the art; even though the error of other practitioners is more prone to harm than that of the astrologer. For the fallacy of a physician often becomes the cause of death; while the failure of a navigator or the error of an astrologer is often followed by the greatest reproach in science. Therefore, let a greater benefit follow from the truth of this art and less harm from its fallacy. And it is not fitting for anyone to disparage the art; and those who profess it must strive greatly so that the innocent art is not rendered hateful because of anyone's personal fault.

Chapter Two. Concerning the effect of the movement of the stars.

A woodcut depicts the letter 'N' enclosed in a square frame, decorated with intricate floral and foliate vine patterns.

Now it is fitting to speculate on the effects of the stars, having premised what must be done in every treatise: that one should arrange at the beginning that which is to be discussed. We follow from the beginning of the treatise the guidance of the stars throughout the inferior world i.e., the terrestrial sphere for the generation and corruption of all things, following with a speculation on the nature of the stars and every habit and affection. We are confined by four terms that encompass the knowledge of all things. The first is that by which one seeks whether the thing under discussion exists or not. The second is what it is. The third is what kind it is. The fourth is why it is. The origin of all discovery is first in the senses; for the sense prompts opinion. From this, one ascends to the reason of the intellect. Therefore, as we have taught, the first cause of the discovery of astrology is sight; the second is the reason that followed, so that from here the perfection of the art might ascend to another, certain intellect of the celestial secret. Indeed, it is established by the authority and reason of all philosophy and higher inquiry that the substance of the celestial bodies is neither made from any element of this world nor amassed from several or all of them. For if it were from these elements, the necessities of elemental offspring would follow it: generation, corruption, augmentation, diminution, resolution, and other alterations of that kind; which, since they are alien to the celestial circles and the bodies of the stars, reason concludes that their substance consists of a certain fifth nature a reference to the quintessence or ether. But the quality of those bodies is in their form. For they are spherical, translucent bodies existing in natural motion. We understand the necessary cause of these motions to be so that the motion of the superior essence might mix the inferior natures by acting upon them. This commixture was necessary for all generation. Whence the philosopher finally understood that the inferior world is bound to the superior by a certain necessity, which, having been set in motion by a certain natural movement, drew along what it held. For the superior world, perpetually surrounding the inferior while it draws that which is bound to it, agitates the materials of the world, mixing the acts and passions of the causes of all generations. But a motion other than circular was not suitable for celestial bodies. For it is not perfect unless it is circular, which, since it lacks a cause, a beginning, and an end, does not admit rest in any part; for other motions, since they have a beginning and an end to which they arrive, must necessarily stop. But there are two motions of the body of the inferior world, the other being straight and having an end to which...