In his comprehensive exposition of the Tenth Commandment (Spec. 4.79131), Philo considers the prohibition You shall not desire: what sort of desire it prohibits (and why) and how the Mosaic dietary laws collectively enforce that prohibition. This volume offers the first complete study of Philos exposition, beginning with an overview of its content, context, and place in previous research. In-depth studies of Philos concept of desire and his concept of self-control provide background and demonstrate Philos fundamental agreement with contemporary Middle-Platonic moral psychology, especially in his theory of emotion (pathos). A new translation of the exposition, with commentary, offers a definitive explanation of Philos view of the Tenth Commandment, including precisely the sort of excessive desire it targets and how the dietary laws work as practical exercises for training the soul in self-control.