When Leo XII promulgated Aeterni Patris in 1879, he stipulated that the “Leonine,” or official, edition of the Summa should always be printed in conjunction with Cajetan’s Commentary. For five hundred years they were studied together. Generations were trained by reading through the Summa article by article with Cajetan’s commentaries in hand. Early printed editions of the Summa typically included them in a Talmudic arrangement, as marginal text running around each article by Aquinas. This edition imitates that example. Recently, serious thinkers of all denominations – and none – have found new reasons to be interested in St. Thomas. His text is deceptively simple, yet important issues are handled in every article, sometimes below the surface. Cajetan extracts these hidden issues, and explains and elaborates on them with remarkable affinity to modern analytical philosophy. Part of that affinity lies in the use of modal logic, a tool whose importance was overlooked between the Renaissance and the twentieth century. The time is ripe for an analytically-inspired translation of Thomas: hence this volume. Never until now has Cajetan’s Commentary been put into English in its entirety. William Marshner’s translation is consistent with fidelity to the technical force of the original. The translator’s footnotes acknowledge what empirical science has made obsolete in the work of St. Thomas, and also make clear how much today’s science would have saved Thomas useless labor. This volume will, for the first time, make Cajetan’s help available to the modern reader.