The examination of the Aristotelian and the Stoic theory of categories led Plotinus, a Platonist, to develop a two-level theory of categories which differentiates between the categories of thought and the categories of the visible world. The fact that the way in which the Aristotelian concept of categories viewed the issues was able to prevail over Plotinus‹s approach is evidence in particular of the comprehensive work of the Neoplatonist Simplicius, whose commentary on the Aristotelian categories refutes Plotinus‹ criticism of Aristotle‹s theory. Simplicius‹ commentary, which became known due to the translation by Wilhelm von Moerbeke, played an important role in the medieval discussion about the theory of categories. In the 16th century, Simplicius‹ commentary also became known due to the new translation by Giullelmus Dorotheus, which is reprinted here.