In Proclus' penetrating exposition of Euclid's methods and principles, the only one of its kind extant, we are afforded a unique vantage point for understanding the structure and strength of the Euclidean System. A primary source for the history and philosophy of mathematics, Proclus' treatise contains much priceless information about the mathematics and mathematicians of the previous seven or eight centuries that has not been preserved elsewhere. This is virtually the only work surviving from antiquity that deals with what we today would call the philosophy of mathematics. To all the students interested in the logic and history of mathematics and in the relations between philosophy and mathematics in antiquity, this volume will be an invaluable resource. In his new forward, Ian Muller discusses new scholarship on the commentary and places the work in historical and cultural context.