ABSTRACTThere has been much debate surrounding the way in which Ptolemy handled the observations that are behind his planetary models. This article aims at presenting an interesting case of manipulation of observational data within the epistemological frame of what we may call Ptolemy’s philosophy of knowledge. It deals with an interesting case from the Almagest, that of the determination of the longitudes of the apogee and perigee of Venus’s deferent, and shows that the selections and adjustments Ptolemy carried out in (...) order to obtain the needed results are nothing but an analogous application of the epistemological doctrines expressed in his On the Kriterion and in his Harmonics. (shrink)