The first and central task of this chapter is to examine the relevant passages and offer a coherent interpretation of Aristotle’s explanation of the deceptive character of dreams. Apart from furthering our understanding of Aristotle, coming to grips with this task is fundamental for an appreciation of the ways in which the subject of dreaming is approached in the Aristotelian tradition. Not only will the chapter introduce some crucial concepts that will recur in the following chapters of this volume – such as the common sense, appearance, belief – but it will also give the reader a sense of the diversity and plasticity of the Aristotelian tradition. The reader has to understand Aristotle’s views to be able to see just how different, and even opposite, views on the same subjects have been entertained by later thinkers who were influenced by Aristotle or indeed who considered themselves followers of Aristotle. This brings me to my second task, which is to present the interpretation of Aristotle’s explanation of the deceptive character of dreams proposed by the Byzantine scholar Michael of Ephesus (1050–1129) in his commentary on Aristotle’s De insomniis. Michael’s is the only extant Greek commentary, and it is the earliest commentary that we have, in any language, on any of Aristotle’s three short treatises on sleep and dreams. The way Michael reads Aristotle’s text and the way he updates Aristotle’s physiology of sleeping and dreaming is a fine example of the plasticity of the Aristotelian tradition.