How can there be evil in a providential world? Plato , as so often, set the problem, without bequeathing to his commentators any one solution. In three places, he insists that God is not responsible (anaitios ) for evil, Timaeus 42D; Republic 2, 379C; Republic 10, 617E. In the last passage, he exonerates God from causing sin by the device of letting souls choose their next incarnation. As for responsibility for imperfections other than human action, Plato blames the recalcitrance of matter in the Timaeus, e.g. 75A. Again, at Theaetetus 176A6, he suggests that good needs evil as its contrary.
Aristotle ’s ideas on chance and on anomalies in nature came to be exploited, we shall see, as solutions. Aristotle further argues in Metaphysics 9, 1051a17-21, that evil does not exist (like Platonic Forms) apart from individual things, and that there is no evil among the principles of the universe (such as the Indefinite Dyad , postulated orally by Plato in his lecture on the Good, ap. Aristotelem, Metaph. 13.6-7, or the evil soul which some Platonists detected in Plato’s universe).
But for Aristotelians the problem was less acute, because they took it that Aristotle would deny that God’s providence concerned itself with individuals in the world beneath the heavens. The Epicureans were more extreme and denied that the gods had any concern with the world at all. By contrast, Plato Laws 905A and Plotinus in his treatise on providence 3.2-3 [47-8], extend providence to many individual things. But Plotinus still denies the gods have knowledge of every detail. Proclus , by contrast, makes deity omniscient.
The Stoics’ deity knows all things (but see Cicero ND 2.167; Plutarch St. Rep. 1051C on his neglect of small things). They regarded vice as the only genuine evil, but still sought to explain other apparent imperfections. They therefore needed an explanation of these imperfections as well as of genuine evil. They sometimes sought to deny that local evil was really evil in the universe taken as a whole. For example, they sometimes said (Plutarch Com. Not. 1065D-E; Marcus Aurelius 6.42, both in SVF 2.1181), that evil is needed for the whole, as a mixture of black and white or of high and low notes is needed in the visual arts and music. The Platonist Plutarch of Chaeroneia, followed by Augustine , endorsed this. The Stoics also made a suggestion which we shall see revived later, that evil is an accidental effect, On the Generation of Soul 1015B-C. Plutarch, however, rejects this.
As regards the explanation of evil detected in Plato by some Middle Platonists, that it is due to the existence of an evil power, Numenius exploited Plato’s Indefinite Dyad as an evil force. Plutarch claims, inaccurately, that Plato in Laws 896E; 897B; 897D; 898C postulates an evil soul, which Plutarch takes to be responsible for pre-cosmic disorder, On Isis and Osiris 370F; On the Generation of Soul in the Timaeus 1014D-E; 1015E, and to retain some such tendency even after its combination with Intellect to form the cosmic World Soul, since, On the Generation of Soul 1024D, it is connected with the Difference and Divisibility that Timaeus 35A locates in the World Soul. Atticus agrees that Plato postulates an evil soul, ap. Proclum in Tim. 1.381,26-382,12; 1,391,6-12.