If all things ultimately proceed from the One-Good, how can Socrates say that evil occurs by necessity and that it is contrary to the Good? In my lecture I will present how Proclus and Simplicius explain this provocative Platonic text. The necessity of evil is due to the fact that the procession of all things must include things that may change and go wrong. “If the creator had really wanted to create a world where no evil could occur, he would no longer be good.” (Simplicius). All forms of evil originate as defects, shortcomings in beings. They have no being of their own, but owe their reality to the fact that they are present in beings, parasite on them as para-hypostaseis. For that reason, Socrates in Theaetetus calls evil not a contrary to the good, but a subcontrary (hupenantion).