Plotinus argued the position at length in a treatise scholars now number Ennead 2.9, generally known by the title Against the Gnostics, aimed at a group within his own Roman circle whose teaching he treated as a serious enough threat to answer systematically rather than dismiss. Where Gnostic cosmology typically holds that the visible world is the flawed or malicious product of an inferior craftsman, distinct from and opposed to a true, unknowable God above it, Plotinus insists the material cosmos proceeds from the One through Intellect and Soul in an unbroken chain, so the world, however far down the chain of emanation it sits, cannot be the work of a hostile or incompetent second power, since there is no second power for it to be the work of. Evil, on this account, is not a rival substance requiring a rival explanation, it is the diminishing of reality and form that necessarily occurs at emanation's furthest point, matter understood as privation rather than as an opposing thing.
Augustine's testimony, recorded in his own Confessions, credits reading of this kind, encountered in Latin translations of Platonist writers, with helping free him from Manichaean dualism before his conversion to Christianity, and privation became the standard Latin Christian answer to why a good creation contains evil, repeated from Augustine through the medieval scholastics. The argument answers the coherence problem, a single good source need not explain evil as another thing it also made, more cleanly than it answers the practical problem of why privation is experienced as anything at all.