Book excerpt: Aristotle classified the things in the world into ten categories: substance, quantity, quality, relative, etc. Plotinus, the founder of Neoplatonism, attacked the classification, accepting only these first four categories, rejecting the other six, and adding one of this own: change. He preferred Plato's classification into five kinds which included change. In this part of his commentary, Simplicius records the controversy on the six categories which Plotinus rejected: acting, being acted upon, being in a position, when, where, and having on. Plotinus' pupil and editor, Porphyry, defended all six categories as applicable to the physical world, even if not to the world of Platonic Forms to which Platonist studies must eventually progress. Porphyry's pupil, lamblichus, went further: taken in a suitable sense, Aristotle's categories apply also to the world of Forms, although they require Pythagorean reinterpretation. Simplicius may be closer to Porphyry that to lamblichus, and indeed Porphyry's defence established Aristotle's categories once and for all in Western thought. But the probing controversy of this period none the less revealed more effectively than any discussion of modern times the profound difficulties in Aristotle's categorical scheme.