Evil, then, may be of two kinds. When the defect of privation is found in relation to the nature of something we call it natural evil. When it is found in relation to the end of an action accomplished through will is called moral evil.
Actions which are within the power of the actor and whose principles are found within the will are known as voluntary actions. This type of action is distinguished from natural actions, actions done because of some force external to the agent, and actions done without knowledge. All of these actions are involuntary, and their defects do not constitute moral evil.
A defect in voluntary action, though, is a moral evil, and it is known as sin.