Thus we see that it is not universally true that an Essence can have no contrary. Enneads: I. VIII. 6

But if this were so, part of the Soul must possess The Good, part be without it; the Soul will have a mingled nature and the Evil within it will not be unblended: we have not yet lighted on the Primal, Unmingled Evil. The Soul would possess the Good as its Essence, the Evil as an Accidental. Enneads: I. VIII. 10