Pain here means "penalty": such was its original signification, being derived from "poena." In this sense we say "Pain of death, Pain of loss, Pain of sense."--Ed.
Objection 1. It would seem that evil is not adequately divided into pain and fault. For every defect is a kind of evil. But in all creatures there is the defect of not being able to preserve their own existence, which nevertheless is neither a pain nor a fault. Therefore evil is inadequately divided into pain and fault.
Objection 2. Further, in irrational creatures there is neither fault nor pain; but, nevertheless, they have corruption and defect, which are evils. Therefore not every evil is a pain or a fault.
Objection 3. Further, temptation is an evil, but it is not a fault; for "temptation which involves no consent, is not a sin, but an occasion for the exercise of virtue," as is said in a gloss on 2 Corinthians 12; not is it a pain; because temptation precedes the fault, and the pain follows afterwards. Therefore, evil is not sufficiently divided into pain and fault.
Objection 4.On the contrary, It would seem that this division is superfluous: for, as Augustine says (Enchiridion 12), a thing is evil "because it hurts." But whatever hurts is penal. Therefore every evil comes under pain.
I answer that, Evil, as was said above (Article 3) is the privation of good, which chiefly and of itself consists in perfection and act. Act, however, is twofold; first, and second. The first act is the form and integrity of a thing; the second act is its operation. Therefore evil also is twofold. In one way it occurs by the subtraction of the form, or of any part required for the integrity of the thing, as blindness is an evil, as also it is an evil to be wanting in any member of the body. In another way evil exists by the withdrawal of the due operation, either because it does not exist, or because it has not its due mode and order. But because good in itself is the object of the will, evil, which is the privation of good, is found in a special way in rational creatures which have a will. Therefore the evil which comes from the withdrawal of the form and integrity of the thing, has the nature of a pain; and especially so on the supposition that all things are subject to divine providence and justice, as was shown above (Question 22, Article 2); for it is of the very nature of a pain to be against the will. But the evil which consists in the subtraction of the due operation in voluntary things has the nature of a fault; for this is imputed to anyone as a fault to fail as regards perfect action, of which he is master by the will. Therefore every evil in voluntary things is to be looked upon as a pain or a fault.
Reply to Objection 1. Because evil is the privation of good, and not a mere negation, as was said above (Article 3), therefore not every defect of good is an evil, but the defect of the good which is naturally due. For the want of sight is not an evil in a stone, but it is an evil in an animal; since it is against the nature of a stone to see. So, likewise, it is against the nature of a creature to be preserved in existence by itself, because existence and conservation come from one and the same source. Hence this kind of defect is not an evil as regards a creature.
Reply to Objection 2. Pain and fault do not divide evil absolutely considered, but evil that is found in voluntary things.
Reply to Objection 3. Temptation, as importing provocation to evil, is always an evil of fault in the tempter; but in the one tempted it is not, properly speaking, a fault; unless through the temptation some change is wrought in the one who is tempted; for thus is the action of the agent in the patient. And if the tempted is changed to evil by the tempter he falls into fault.
Reply to Objection 4. In answer to the opposite argument, it must be said that the very nature of pain includes the idea of injury to the agent in himself, whereas the idea of fault includes the idea of injury to the agent in his operation; and thus both are contained in evil, as including the idea of injury.
Source: Summa Theologica (New Advent)