This chapter focuses on Hypatia of Alexandria and the school of thought she represented in fourth-century CE Alexandria. It follows the inverse chronological order and characterizes in turn the works and thoughts of Hypatia, Theon and Pappus of Alexandria, before concluding with a short discussion of the fact that they do not seem to have cultivated the 'observational' and 'physical' sides of Ptolemy's doctrine. It is often taken for granted that Hypatia was a philosopher in the Platonic tradition on the basis of three main arguments. The first is Socrates Scholasticus' testimony, the only source asserting that she took the succession of the Platonic school 'from Plotinus'. The second is a generous inference from the contents of Synesius' philosophy to that of his teacher. The last is a kind of 'contamination' argument, according to which she must have been a Platonist because the major trend of philosophy in this period is Platonism and because this philosophy was compatible with Christianity.