 Augustine’s notion of will is closely related to his thinking on evil. The problem of the origin of evil (unde malum), he claims, had haunted him from his youth (Confessiones 7.7). At first, he accepted the dualist solution of the Manicheans, which freed God from the responsibility for evil but compromised his omnipotence (ib. 3.12; 7.3). After having encountered the books of the Platonists, Augustine rejected the existence of an evil substance and endorsed the Neoplatonic view (argued e.g., in Plotinus, Enneads I.8) that evil is in fact unsubstantial and a privation or corruption of goodness. In his mature view, which was largely developed during his anti-Manichean polemics, everything that has being is good insofar as it has been created by God. There are of course different degrees of goodness as well as of being (Letter 18.2),  but everything that is real is good “in its degree”, and the hierarchical order of reality is itself a good creation of God (Bouton-Touboulic 2004). Augustine therefore rejects Plotinus’ view that prime matter is equivalent to prime evil, because the formlessness of matter is not pure negativity but a positive, and hence divinely created, capacity to receive forms (Confessiones 12.6; see  10. Creation and Time).  A created being can be said to be evil if and only if it falls short of its natural goodness by being corrupted or vitiated; strictly speaking, only corruption itself is evil, whereas the nature or substance or essence (for the equivalence of the terms see De moribus 2.2) of the thing itself remains good (Confessiones 7.18; Contra epistulam fundamenti 35.39 etc.; for a systematic account, De natura boni 1–23; Schäfer 2002: 219–239). While this theory can explain physical evil relatively easily either as a necessary feature of hierarchically ordered (corporeal) reality (De ordine 2.51), as a just punishment of sin, or as part of God’s pedagogy of salvation (Letter 138.14), it leaves open the question of moral evil or sin itself. Augustine answers by equating moral evil with evil will  and claims that the seemingly natural question of what causes evil will is unanswerable. His most sustained argument to this effect is found in his explanation of the fall of the devil and the evil angels, a case that, being the very first occurrence of evil in the created world, allows him to analyze the problem in its most abstract terms (De civitate dei 12.1–9; cf. already De libero arbitrio 3.37–49; Schäfer 2002: 242–300; MacDonald 1999). The cause can neither be a substance (which, qua substance, is good and unable to cause anything evil) nor a will (which would in turn have to be an evil will in need of explanation). Therefore, an evil will has no “efficient” but only a “deficient” cause, which is none other than the will’s spontaneous defection from God. The fact that evil agents are created from nothing and hence are not, unlike God, intrinsically unable to sin is a necessary condition of evil but not a sufficient one (after all the good angels successfully kept their good will). In this context Augustine, in an interesting thought experiment, imagines two persons of equal intellectual and emotional disposition of whom one gives in to a temptation while the other resists it; from this he concludes that the difference must be due to a free, spontaneous and irreducible choice of the will (De civitate dei 12.6). Here at least Augustine virtually posits the will as an independent mental faculty.