One of the earliest 13th-century Latin commentators on Aristotle, Robert Kilwardby (d.1279) is an original logician and an ingenious interpreter. This is the first full-length study of his Prior Analytics commentary, and the first study to work from the medieval manuscripts. Kilwardby interprets Aristotle’s syllogistic within a broad ontological context that includes the four causes, and concepts of degrees of perfection. His interpretation aims to preserve Aristotle’s theses as correct; and he formulates semantic and syntactic hypotheses that achieve this aim with almost total perfection. The book includes an appendix offering a modern rendering of Kilwardby’s original logical ideas.