  Since the heavenly bodies were considered divine, because they are eternal and living beings, the study of the heavens was of special importance to Neoplatonists. In the preface to his treatise On Astronomical Hypotheses (a summary and evaluation of astronomical views of his time), Proclus makes it clear that his approach is based on Plato’s remarks on astronomy (especially in the Republic and in the Laws). He feels the need to go through the different theories, because one can observe a great disagreement among ancient astronomers on how to explain the different phenomena (Hyp. I § 33). Fundamental to Proclus’ approach is the distinction between two kinds of astronomy (Hyp. I § 1–3). The first kind contents itself with observing the heavenly phenomena and formulating mathematical hypotheses to explain them and make calculations and prognostics possible. This is the astronomy as practiced by the most famous astronomers before Proclus’ time (Aristarchus, Hipparchus, and Ptolemy). The second, which is developed by Plato in the Timaeus, and is confirmed by the tradition of the “Chaldaeans and Egyptians,” investigates into the intelligible causes of heavenly movements. An example for this approach can be found in his Commentary on Plato’s Republic (In Remp. II 227.23–235.3). There, Proclus explains that the seemingly irregular movements of the planets ought not to be explained by means of Ptolemy’s complicated theory of excentric spheres and epicycles, but are rather due to the fact that the planets are moved by intelligent souls which express in the movements of their bodies “the invisible powers of the Forms” (232.1–4). Yet Proclus appreciates Ptolemy’s astronomy as long as it is seen only as a mathematical-mechanical construction making it possible to calculate and predict the positions of planets, and as long as it does not claim to have any real explanatory value. For the history of astronomy Proclus’ Astronomical Hypotheses remains a most valuable document, since it represents one of the best introductions to Ptolemy’s Almagest extant from antiquity and since it explains the most important ancient astronomical theories, in order finally (in chapter seven of the work) to evaluate them critically. Proclus’ arguments also played an important role in the scientific discussion of the Ptolemaic hypotheses in the 16th and 17th century.