For injustice is not the kind of thing which infects and corrupts the soul in the way that poison infects and corrupts the body; nor does it do something in the way that happens when a wicked man does evil deeds. When a savage beast breaks its bonds and rages about wildly, and when a ship—if the helmsman leaves the rudder and delivers the vessel to the wind and the waves—strays and is driven into dangers of one kind or another, we say that the absence of chains or of a rudder causes these events. [We say this] not because their absence is something or does something but because if they had been present they would have caused the wild animal not to rage and the ship not to perish. By comparison, when an evil man rages and is driven into various dangers to his soul, viz., evil deeds, we declare that injustice causes these deeds. [We say this] not because injustice is a being or does something but because the will (to which all the voluntary movements of the entire man are submitted), lacking justice, driven on by various appetites, being inconstant, unrestrained, and uncontrolled, plunges itself and everything under its control into manifold evils—all of which justice, had it been present, would have prevented from happening.
[Anselm of Canterbury, De Conceptu Virginali (PDF), Chapter 5, Jasper Hopkins, tr.]