To absolve God of authoring evil, Plutarch dramatically re-reads Plato's Timaeus. While most Middle Platonists leaned toward a monistic framework where evil arises as a privation or byproduct of matter's distance from the divine One, Plutarch posits a more radical solution: a robust, ontological dualism. He argues for two primary, opposing principles present before the cosmos was ordered: the divine Monad, the source of all good and order, and the Indefinite Dyad, an active, malevolent principle of disorder and multiplicity. This framework is most explicitly developed in his treatises De Iside et Osiride and De animae procreatione in Timaeo.

Plutarch's fundamental problem is explaining cosmic recalcitrance—the persistent existence of disorder, strife, and evil in a world created by a benevolent Demiurge. A purely monistic system, in his view, either implicates God in the creation of evil or fails to adequately account for its potent reality. His solution is to locate evil in an independent, primordial cause that exists alongside, and in opposition to, God.

Plutarch's most significant exegetical move is his interpretation of Timaeus 35a. Where others saw the creation of the World Soul ex nihilo or from abstract principles, Plutarch argues that the Demiurge constructs it from a pre-existing substrate: a disorderly, irrational soul. This entity is the embodiment of the Indefinite Dyad. It is not mere matter (hyle), but a dynamic, active principle of chaos.

The act of creation, then, is not about forming a world from passive, inert matter. Instead, it is the process of the Demiurge imposing reason (Logos) upon this unruly, pre-cosmic soul. By introducing number, harmony, and proportion, the divine intellect transforms this chaotic entity into the orderly, rational World Soul. The formula for the World Soul's creation in the Timaeus is thus a description of this transformation.

However, the victory of reason is not absolute. The original chaotic nature of the soul is never entirely eradicated; it is only subdued. This residual element of the Malign Soul persists within the ordered cosmos, explaining the continued existence of evil, decay, and irrationality in the physical world. God is the author of all good, but evil originates from this independent, recalcitrant principle that was merely tamed, not annihilated.

This dualistic model sets Plutarch apart from his contemporaries. Standard Middle Platonism explained evil as a deficiency resulting from matter's inability to perfectly receive the Forms. In this view, matter is passive and evil is a privation of good (steresis), not a positive force. Plutarch rejects this, arguing that privation alone cannot account for the active power of evil he observes.

By positing an active, malevolent source of evil, he creates a metaphysical battleground where order is perpetually imposed upon an inherently disorderly force. His God is not an omnipotent creator in the later Christian sense, but a divine craftsman (Demiurge) who brings order to a pre-existing chaos that inherently resists him. This interpretation, while controversial, provides a powerful explanation for the problem of theodicy by establishing a clear and distinct origin for cosmic evil, fully separate from the divine Monad.

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