This is the first complete English translation of On the Purity of the Art of Logic, a handbook of logic written in Latin by English philosopher Walter Burley (c. 1275-1344/5). The work circulated in the Middle Ages in two versions, a shorter and a longer one, both translated here by Paul Vincent Spade. The translations arc based on the only complete edition of Burley's treatises, corrected by Spade on the basis of one of the surviving manuscripts. The book also includes an extensive introduction, explanatory notes, a table of corresponding passages between the two versions, a select annotated bibliography, and three indexes. A contemporary of John Duns Scotus and William of Ockham, Burley was active at the universities of both Paris and Oxford. He became one of the most important figures in the transformation of medieval logic and semantics that took place in the early fourteenth century. Burley used new tools and techniques of logical and semantical analysis, yet in many cases he used them in defense of traditional views, such as a realist metaphysical theory of "universals". On the Purity of the Art of Logic shows both these sides of Burley -- the innovator and the conservative -- as well as some of the ways in which his views corresponded or clashed with those of William of Ockham.