Plotinus’ polemical treatise Against the Gnostics has proved remarkably resistant to modern methods of criticism. So much so, that historians of philosophy frequently end up attributing to Plotinus himself the very beliefs which he attempts to demolish in his work. In this work, 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 new understanding of Plotinus’ theodicy and of the way in which Plotinus conceived his relation to the Gnostics. O'Brien’s analysis supports the view that a consistent and original philosophy underlies that complexities and obscurities of the text of the Enneads .