Ptolemy's Tetrabiblos

The studies preliminary to astronomical prognostication, O Syrus! are two: the one, first alike in order and in power, leads to the knowledge of the figurations of the Sun, the Moon, and the stars; and of their relative aspects to each other, and to the earth: the other takes into consideration the changes which their aspects create, by means of their natural properties, in objects under their influence.
The first mentioned study has been already explained in the Syntaxis to the utmost practicable extent; for it is complete in itself, and of essential utility even without being blended with the second; to which this treatise will be devoted, and which is not equally self-complete. The present work shall, however, be regulated by that due regard for truth which philosophy demands: and since the material quality of the objects acted upon renders them weak and variable, and difficult to be accurately apprehended, no positive or infallible rules (as were given in detailing the first doctrine, which is always governed by the same immutable laws) can be here set forth: while, on the other hand, a due observation of most of those general events, which evidently trace their causes to the Ambient, shall not be omitted.