John Buridan (ca. 1300-1361) was one of the most influential  philosophers of his time. During his long career at the Faculty of Arts at the University of Paris, he taught many logic courses, for which he wrote a textbook, entitled Summulaedialecticae. This work consists of nine treatises; the present volume contains the first critical edition of the Preface and the first treatise of the Summulae: De propositionibus. As the bearers of truth and falsity, propositions are the primary concern of logic, the art that serves as a general tool for reaching truth and avoiding falsity in any field of knowledge, whether in contemplative or practical contexts. Most important is Buridan’s commitment to the semantic primacy of mental language and the treatment of written and spoken propositions as conventional signs, which designate the primary bearers truth and falsity, namely mental propositions. In De propositionibus Buridan develops his nominalist conception of the relations between mind, language, and reality, which he goes on to employ in the subsequent treatises of the Summulae.