Klaudios Ptolemaios, or Ptolemy, is known today mainly for his contributions to astronomy and astrology. Following Otto Neugebauer, historians of science tend to emphasize the significance of Ptolemy’s astronomical models. These models clearly deserve this attention, for they served as the western world’s paradigm in astronomy for approximately 1,400 years, up to the time of the Scientific Revolution. Modern astrologers, on the other hand, continue to hold Ptolemy’s astrological work, the Tetrabiblos, in high regard. If Ptolemy is known principally for his mathematical and natural philosophical contributions, one may reasonably wonder, why does he deserve a place in this volume on philosophy in late antiquity?

The answer, put plainly, is that Ptolemy was a self-identified philosopher who examined a number of the most pressing philosophical questions of his time, commented on the (lack of) success of previous philosophical theories, appropriated the philosophical concepts of contemporary schools, and, moreover, propounded philosophical ideas unprecedented in the history of ancient Greek philosophy. Ptolemy, however, was not a typical philosopher. He neither affiliated himself with a specific school nor did he proclaim himself an eclectic, as did his contemporary Galen. Ptolemy’s texts, in fact, reveal him to be a Platonic empiricist. He adopts Platonic, Aristotelian, and, to a lesser extent, Stoic ideas, but the manner in which he mixes these philosophical influences depends heavily on contemporary Platonic concerns. While Ptolemy does not identify himself as a Platonist in his texts, the ideas he promulgates reveal a substantial Platonic influence on his philosophy.