Laura M. Castelli presents a new translation and comprehensive commentary of the tenth book (Iota) of Aristotle’s Metaphysics, which provides Aristotle’s most systematic account of what it is for something to be one, what it is for something to be a unit of measurement, what contraries are, and what the function of contraries is in shaping the structure of reality into genera and species. There are some objective difficulties in making sense of Iota as a part of the Metaphysics and as a piece of Aristotelian philosophy. Castelli’s Introduction tackles such general difficulties, while the commentary provides a detailed analysis of the arguments, of the more specific issues and of the philosophical points emerging from Aristotle’s text. The English translation, based on Ross’ critical edition, is meant as a tool for readers with or without knowledge of ancient Greek.