This paper argues that research in the primary sources must precede the investigation of Byzantine philosophy. Two points are to be considered, on the one hand, the gathering of texts, and, on the other hand, the study of texts in relation to their sources. Thus the external evidence as well as the internal evidence of texts should be examined. In this double regard, the manuscripts containing Aristotle’s Nicomachean Ethics are considered. Their authors are Michael of Ephesos, Eustratios of Nicaea, “Anonymus”, Heliodoros of Prussa, Georgios Pachymeres, Michael Psellos, John Italos, Nikephoros Blemmydes, George Gemistos Plethon.