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. [Sidenote: 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. [Sidenote: Astrology as natural science.]