Plotinus' fiercely polemical treatise Against the Gnostics has proved peculiarly resistant to modern methods of criticism. So much so, that historians of philosophy frequently end up attributing to Plotinus himself the very beliefs which Plotinus attempts to demolish in his criticism of the Gnostics. Denis O'Brien attempts to unravel this paradox by showing that, in earlier treatises of the Enneads, Plotinus puts forward a theory of the generation of matter by soul, which he then takes for granted in his attack on the Gnostics. This leads to a wholly new understanding of...