 This tradition began with the prooemium to Porphyry’s Isagoge, in which he explicitly refused to discuss whether genus and species depend on thought alone, are incorporeal or inseparable, as a realist interpretation of universals would have it—that discussion should be left, he said, to a “greater work”. The medieval tradition followed Boethius in reading the Isagoge as an introduction to Aristotle’s Categories, and took Porphyry’s skirting of the ontological status of genera and species as an invitation to explore universals. Boethius himself adopted a moderate realist position on universals in his second commentary on the Isagoge. There he argued that the mind abstracts universals from concrete reality: abstraction involves gathering or collecting the likeness (similitudo) of concrete, individual things. Boethius claimed that, understood in this way, universals have a certain kind of separate, incorporeal reality: