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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 1 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.
It is, however, a common practice with the vulgar to slander everything which is difficult of attainment, and surely they who condemn the first of these two studies must be considered totally blind, whatever arguments may be produced in support of those who impugn the second. There are also persons who imagine that whatever they
themselves have not been able to acquire, must be utterly beyond the reach of all understanding; while others again will consider as useless any science of which (although they may have been often instructed in it) they have failed to preserve the recollection, owing to its difficulty of retention. In reference to these opinions, therefore, an endeavour shall be made to investigate the extent to which prognostication by astronomy is practicable, as well as serviceable, previously to detailing the particulars of the doctrine.
1:1 The Almagest, or Magna Constructio.