A History of Magic and Experimental Science, Volume 1 (of 2) / During the First Thirteen Centuries of Our Era

by Lynn Thorndike

In the Tetrabiblos Ptolemy first engages in argument as to the validity of the art of judicial astrology. If his remarks in this connection were not already trite contentions, they soon came to be regarded as truisms. The laws of astronomy are beyond dispute, says Ptolemy, but the art of prediction of human affairs from the courses of the stars may be assailed with more show of reason. Opponents of astrology object that the art is uncertain, and that it is useless since the events decreed by the force of the stars are inevitable. Ptolemy opens his argument in favor of the art by assuming as evident that a certain force is diffused from the heavens over all things on earth. If ignorant sailors are able to judge the future weather from the sky, a highly trained astronomer should be able to predict concerning its influence on man. The art itself should not be rejected because impostors frequently abuse it, and Ptolemy admits that it has not yet been brought to the point of perfection and that even the skilful investigator often makes mistakes owing to the incomplete state of human science. For one thing, Ptolemy regards the doctrine of the nature of matter held in his time as hypothetical rather than certain. Another difficulty is that old configurations of the stars cannot safely be used as the basis of present day predictions. Indeed, so manifold are the different possible positions of the stars and the different possible arrangements of terrestrial matter in relation to the stars that it is difficult to collect enough observations on which to base rules of general judgment. Moreover, such considerations as diversity of place, of custom, and of education must be taken into account in foretelling the future of different persons born under the same stars. But although for these reasons predictions frequently fail, yet the art is not to be condemned any more than one rejects the art of navigation because of frequent shipwrecks.

Influence of the stars not inevitable.

Nor is it true that the art is useless because the decrees of the stars are inevitable. It is often an advantage to have previous knowledge even of what cannot be avoided. Even the prediction of disaster serves to break the news gently. But not all predictions are inevitable and immutable; this is true only of the motion of the sky itself and events in which it is exclusively concerned. “But other events which do not arise solely from the sky’s motion, are easily altered by application of opposite remedies,” just as we can in part remedy the hurt of wounds and diseases or counteract the heat of summer by use of cooling things. The Egyptians have always found astrology useful in the practice of medicine.

Astrology as natural science.

Ptolemy next proceeds to set forth the natures and powers of the stars “according to the observations of the ancients and conformably to natural science.” Later, when he comes to the prediction of particulars, he still professes “to follow everywhere the law of natural causation,” and in a third passage he states that he “will omit all those things which do not have a probable natural cause, which many nevertheless scrutinize curiously and to excess: nor will I pile up divinations by lot-castings or from numbers, which are unscientific, but I will treat of those which have an investigated certainty based on the positions of the stars and the properties of places.” Connecting the positions of the stars with earthly regions,—it is an art that fits in well with Ptolemy’s other occupations of astronomer and geographer! The Tetrabiblos has been called “Science’s surrender,”[514] but was it not more truly divination purified and made scientific?

Properties of the planets.

Taking up first the properties of the seven planets, Ptolemy associates with each one or more of the four elemental qualities, hot, cold, dry, and moist. Thus the sun warms and to some extent dries, for the nearer it comes to our pole the more heat and drought it produces. The moon is moist, since it is close to the earth and is affected by the vapors from the latter, while its influence renders other bodies soft and causes putrefaction. But it also warms a little owing to the rays it receives from the sun. Saturn chills and to some extent dries, for it is remote from the sun’s heat and earth’s damp vapors. Mars emits a parching heat, as its color and proximity to the sun indicate. Jupiter, situated between cold Saturn and burning Mars, is of a rather lukewarm nature but tends more to warmth and moisture than to their opposites. So does Venus, but conversely, for it warms less than Jupiter does but moistens more, its large surface catching many vapors from the neighboring earth. In Mercury, situated near sun, moon, and earth alike, neither drought nor dampness predominates, but the velocity of that planet makes it a potent cause of sudden changes. In general, the planets exert a good or evil influence as they abound in the two rich and vivifying qualities, heat and moisture, or in the detrimental ones, cold and drought. Wet stars like the moon and Venus, are feminine; Mercury is neuter; the other planets are masculine. The sex of a planet may also, however, be reckoned according to its position in relation to the sun and the horizon; and changes in the influences exerted by the planets are noted according to their position or relation to the sun. This discussion of the properties of the planets is neither convincing nor scientific. It seems arguing in a circle to make their effects upon the earth depend to such an extent upon themselves being affected by vapors from the earth. Indeed we are rather surprised that an astronomer like Ptolemy should represent vapors from the earth as affecting the planets at all. But his discussion is at least an effort, albeit a feeble one, to express the potencies of the planets in physical terms.

Remaining contents of Book One.

Ptolemy goes on to discuss the powers of the fixed stars which seem to depend upon their positions in constellations and their relations to the planets. Then he treats of the influence of the four seasons of the year and four cardinal points, each of which he relates to one of the four qualities, hot, cold, dry, and moist. With a discussion of the signs of the zodiac and their division into Houses and relation in Trigones or Triplicitates or groups of three connected with the four qualities, of the exaltation of the planets in the signs and of other divisions of the signs and relations of the planets to them, the first book ends.

Book Two: Regions.