The paper explores the historical reasons why Ptolemy’s Almagest was cultivated in Antiquity as a legitimate part of ancient culture. It tries to elucidate first the precise nature of Ptolemy’s project in the Almagest, its relation to ethics as well as to the particular conception that Ptolemy had of mathematics. It then proposes an explanation for the fact that this treatise, both in Ptolemy's period and afterwards, was studied within the milieu of ancient astrologers.