  In the sequence of courses, the ‘Prolegomena’ were followed by lectures on Porphyry’s seminal ‘Introduction to Aristotle’s Categories’. Porphyry, who started the neoplatonic tradition of commentaries on Aristotle at the end of the 3rd century, was once approached by a Roman senator, Chrysarius, who had great difficulty understanding Aristotle’s ‘Categories’; Elias and David tell their students how Porphyry seized the opportunity to write what should turn out to be an extremely popular and influential text, the so-called ‘Isagogê’ (‘Introduction’). The text deals with a detailed explanation of the five logical-ontological key concepts: genus, species, differentia, essential attribute and accidental attribute.