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In the Isagoge, Porphyry gives a Platonic shift to the ontological system of Aristotle’s Categories and invests the categories with metaphysical significance. Some of the early medieval thinkers – the realists – develop and clarify this metaphysical view. John Scotus Eriugena’s ontological interpretation and use of the Isagoge in his Periphyseon is on this account a textbook case. John Scotus, influenced by Proclus’ late neoplatonism, makes use of conceptual tools stemming from the Isagoge in order to elaborate his philosophical system.