4 8] The Almagest 65 



essentially different from that either, of the early Greeks, 

 such as Pythagoras, or of the controversialists of the i6th 

 and early lyth centuries, such as Galilei (chapter vi.), for 

 whom the truth or falsity of postulates analogous to those 

 of Ptolemy was of the very essence of astronomy and was 

 among the final objects of inquiry. The arguments which 

 Ptolemy produces in support of his postulates, arguments 

 which were probably the commonplaces of the astronomical 

 writing of his time, appear to us, except in the case of 

 the shape of the earth, loose and of no great value. 

 The other postulates were, in fact, scarcely capable of 

 either proof or disproof with the evidence which Ptolemy 

 had at command. His argument in favour of the immo- 

 bility of the earth is interesting, as it shews his clear 

 perception that the more obvious appearances can be 

 explained equally well by a motion of the stars or by a 

 motion of the earth; he concludes, however, that it is 

 easier to attribute motion to bodies like the stars which 

 seem to be of the nature of fire than to the solid earth, 

 and points out also the difficulty of conceiving the earth to 

 have a rapid motion of which we are entirely unconscious. 

 He does not, however, discuss seriously the possibility that 

 the earth or even Venus and Mercury may revolve round 

 the sun. 



The third book of the Almagest deals with the length of 

 the year and theory of the sun, but adds nothing of import- 

 ance to the work of Hipparchus 



48. The fourth book of the Almagest, which treats of 

 the length of the month and of the theory of the moon, 

 contains one of Ptolemy's most important discoveries. We 

 have seen that, apart from the motion of the moon's orbit 

 as a whole, and the revolution of the line of apses, the 

 chief irregularity or inequality was the so-called equation 

 of the centre ( 39, 40), represented fairly accurately by 



rough explanation of economic phenomena, starts with certain simple 

 assumptions as to human nature, which at any rate are more plausible 

 than any other equally simple set, and deduces from them a number 

 of abstract conclusions, the applicability of which to real life has 

 to be considered in individual cases. But the perfunctory discussion 

 which such a writer gives ol the qualities of the "economic man" 

 cannot of course be regarded as his deliberate and final estimate 

 of human nature. 



