This is a study of Proclus' engagement with Aristotle's theory of motion, with a specific focus on Aristotle's criticism of Plato. It refutes the often-held view that Proclus – in line with other Neoplatonists – adheres to the idea of an essential harmony between Plato and Aristotle. Proclus' views on motion, a central concept in his thought, are illuminated by examining his Aristotelian background. The results enhance our view of the reception and authority of Aristotle in late antiquity, a crucial period for the transmission of Aristotelian thought which immensely shaped the later reading of his work. The book also counteracts the commonly held view that late antique philosophers straightforwardly accepted Aristotle as an authority in certain areas such as logic or natural philosophy.