This book by Albert Dietrich presents a critical edition of the Arabic version of a previously lost work by the ancient Greek philosopher Alexander of Aphrodisias. The original Greek text, 'On the Specific Difference' or 'On the Differentia,' was a significant contribution to Aristotelian logic and metaphysics. Dietrich's meticulous scholarship unearths and reconstructs this important text, offering invaluable insights into the transmission of Greek philosophical thought into the Islamic world. It explores concepts of classification, definition, and the essential properties that distinguish one species from another, making it crucial for scholars of ancient philosophy, classical Arabic studies, and the history of science.