In this lecture, Headmaster William Michael of the Classical Liberal Arts Academy guides students through Chapter 3 of Porphyry’s Introduction—a cornerstone of classical logic and medieval philosophy. This chapter examines the concept of Difference, one of the five predicables, explaining how we distinguish one kind of being from another. Students explore Porphyry’s distinctions between common, peculiar, and specific differences, between separable and inseparable, and between essential and accidental features. By studying these principles, students learn how philosophers and theologians—especially St. Thomas Aquinas—used the notion of Difference to understand classification, definition, and the order of creation itself.

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Mr. William C. Michael, O.P.