According to standard solutions to the logical problem of evil, there is some additional feature of the world that is impossible unless there is moral evil. The presence of moral evil permits some added value to the world.
According to added value solutions, if there were no moral evil in the world, there would be no libertarian free will, no greater goods, or no fully built souls. Since there is significant value in these features, the world is better with them and with moral evil than without them and without moral evil.
Most defenses play upon added value, but one defense argues that there is no way to remove moral evil from the world without diminishing the value of the world to an unacceptable degree.
In his book, The Many Faces of Evil, John Feinberg argues that we assume that if God could remove moral evil, everything else would stay the same (in value). We’d still be the same kinds of beings in the same sort of world doing all the same kinds of things. There would be no evil.
Feinberg argues that this assumption is false. Indeed, God would have to make a worse world than the one we have to remove moral evil. Instead of pointing at an added value obtained only if there is moral evil, Feinberg points at a diminished value that would obtain should God remove moral evil.
God has only a few options for removing moral evil, none of them good. He could destroy human beings, remove their desires, or remove all the objects of their desires. The first option isn’t better than what we have now. Nor are either of the other options.
Desires may give birth to evil deeds, but they are necessary for ordinary survival too. We couldn’t get rid of one sort without getting rid of the other. After all, sinful desires are extensions of non-sinful desires. Envy, gluttony, and laziness are wrongful forms of ordinary desires for goods, food, and rest.
Nor could God remove the objects of sinful desires. After all, to remove the object of the desire of the adulterer would require eliminating someone else’s wife! God couldn’t remove all the objects of sinful desires because that would likely include most ordinary objects of non-sinful desires.
God could intervene every time someone feels like sinning. But think of such a world. Any time people had sinful desires, God would stop them from doing anything. The sheer quantity of interventions would bring the world to a virtual standstill. In all likelihood, the world would cease to function. We couldn’t get anything done.
Consequently, it isn’t that moral evil permits the addition of something with added value that justifies God in allowing moral evil. Instead, it is that getting rid of moral evil would entail making a world in which there are greater evils. Hence, God has a morally sufficient reason for permitting moral evil in this world.