The short aporetic essay On First Principles by Theophrastus, thought to have been transmitted as his Metaphysics, is critically edited for the first time on the basis of all the available evidence–the Greek manuscripts and the medieval Arabic and Latin translations—together with an introduction, English translation, extensive commentary, and a diplomatic edition of the medieval Latin translation. This book equally contributes to Graeco-Arabic studies as ancilla of classical studies, and includes the first critical edition of the Arabic translation with an English translation and commentary, a detailed excursus on the editorial technique for Greek texts whose medieval Arabic translations are extant as well as for the Arabic translations themselves, and a complete Greek and Arabic glossary as a blueprint for future lexica.