Tetrabiblos

OF the means of prediction through astronomy, O Syrus, two arathe most important and valid. One, which is first both in orderand in effectiveness, is that whereby we apprehend the aspects ofthe movements of sun, moon, and stars in relation to each otherand to the earth, as they occur from time to time; the second isthat in which by means of the natural character of these aspectsthemselves we investigate the changes which they bring about inthat which they surround. The first of these, which has its ownscience, desirable in itself even though it does not attain the resultgiven by its combination with the second, has been expounded toyou as best we could in its own treatise by the method ofdemonstration. We shall now give an account of the second andless self-sufficient method in a properly philosophical way