Objection 1. It would seem that death is not the punishment of our first parents' sin. For that which is natural to man cannot be called a punishment of sin, because sin does not perfect nature but vitiates it. Now death is natural to man: and this is evident both from the fact that his body is composed of contraries, and because "mortal" is included in the definition of man. Therefore death is not a punishment of our first parents' sin.
Objection 2. Further, death and other bodily defects are similarly found in man as well as in other animals, according to Ecclesiastes 3:19, "The death of man and of beasts is one, and the condition of them both equal." But in dumb animals death is not a punishment of sin. Therefore neither is it so in men.
Objection 3. Further, the sin of our first parents was the sin of particular individuals: whereas death affects the entire human nature. Therefore it would seem that it is not a punishment of our first parents' sin.
Objection 4. Further, all are equally descended from our first parents. Therefore if death were the punishment of our first parents' sin, it would follow that all men would suffer death in equal measure. But this is clearly untrue, since some die sooner, and some more painfully, than others. Therefore death is not the punishment of the first sin.
Objection 5. Further, the evil of punishment is from God, as stated above (I, 48, 6; I, 49, 2). But death, apparently, is not from God: for it is written (Wisdom 1:13): "God made not death." Therefore death is not the punishment of the first sin.
Objection 6. Further, seemingly, punishments are not meritorious, since merit is comprised under good, and punishment under evil. Now death is sometimes meritorious, as in the case of a martyr's death. Therefore it would seem that death is not a punishment.
Objection 7. Further, punishment would seem to be painful. But death apparently cannot be painful, since man does not feel it when he is dead, and he cannot feel it when he is not dying. Therefore death is not a punishment of sin.
Objection 8. Further, if death were a punishment of sin, it would have followed sin immediately. But this is not true, for our first parents lived a long time after their sin (Genesis 5:5). Therefore, seemingly, death is not a punishment of sin.
On the contrary, The Apostle says (Romans 5:12): "By one man sin entered into this world, and by sin death."
I answer that, If any one, on account of his fault, be deprived of a favor bestowed on him the privation of that favor is a punishment of that fault. Now as we stated in I, 95, 1; I, 97, 1, God bestowed this favor on man, in his primitive state, that as long as his mind was subject to God, the lower powers of his soul would be subject to his rational mind, and his body to his soul. But inasmuch as through sin man's mind withdrew from subjection to God, the result was that neither were his lower powers wholly subject to his reason, whence there followed so great a rebellion of the carnal appetite against the reason: nor was the body wholly subject to the soul; whence arose death and other bodily defects. For life and soundness of body depend on the body being subject to the soul, as the perfectible is subject to its perfection. Consequently, on the other hand, death, sickness, and all defects of the body are due to the lack of the body's subjection to the soul.
It is therefore evident that as the rebellion of the carnal appetite against the spirit is a punishment of our first parents' sin, so also are death and all defects of the body.
Reply to Objection 1. A thing is said to be natural if it proceeds from the principles of nature. Now the essential principles of nature are form and matter. The form of man is his rational soul, which is, of itself, immortal: wherefore death is not natural to man on the part of his form. The matter of man is a body such as is composed of contraries, of which corruptibility is a necessary consequence, and in this respect death is natural to man. Now this condition attached to the nature of the human body results from a natural necessity, since it was necessary for the human body to be the organ of touch, and consequently a mean between objects of touch: and this was impossible, were it not composed of contraries, as the Philosopher states (De Anima ii, 11). On the other hand, this condition is not attached to the adaptability of matter to form because, if it were possible, since the form is incorruptible, its matter should rather be incorruptible. On the same way a saw needs to be of iron, this being suitable to its form and action, so that its hardness may make it fit for cutting. But that it be liable to rust is a necessary result of such a matter and is not according to the agent's choice; for, if the craftsman were able, of the iron he would make a saw that would not rust. Now God Who is the author of man is all-powerful, wherefore when He first made man, He conferred on him the favor of being exempt from the necessity resulting from such a matter: which favor, however, was withdrawn through the sin of our first parents. Accordingly death is both natural on account of a condition attaching to matter, and penal on account of the loss of the Divine favor preserving man from death [Cf. I-II, 85, 6].
Reply to Objection 2. This likeness of man to other animals regards a condition attaching to matter, namely the body being composed of contraries. But it does not regard the form, for man's soul is immortal, whereas the souls of dumb animals are mortal.
Reply to Objection 3. Our first parents were made by God not only as particular individuals, but also as principles of the whole human nature to be transmitted by them to their posterity, together with the Divine favor preserving them from death. Hence through their sin the entire human nature, being deprived of that favor in their posterity, incurred death.
Reply to Objection 4. A twofold defect arises from sin. One is by way of a punishment appointed by a judge: and such a defect should be equal in those to whom the sin pertains equally. The other defect is that which results accidentally from this punishment; for instance, that one who has been deprived of his sight for a sin he has committed, should fall down in the road. Such a defect is not proportionate to the sin, nor does a human judge take it into account, since he cannot foresee chance happenings. Accordingly, the punishment appointed for the first sin and proportionately corresponding thereto, was the withdrawal of the Divine favor whereby the rectitude and integrity of human nature was maintained. But the defects resulting from this withdrawal are death and other penalties of the present life. Wherefore these punishments need not be equal in those to whom the first sin equally appertains. Nevertheless, since God foreknows all future events, Divine providence has so disposed that these penalties are apportioned in different ways to various people. This is not on account of any merits or demerits previous to this life, as Origen held [Peri Archon ii, 9: for this is contrary to the words of Romans 9:11, "When they... had not done any good or evil"; and also contrary to statements made in I, 90, 4; I, 118, 3, namely that the soul is not created before the body: but either in punishment of their parents' sins, inasmuch as the child is something belonging to the father, wherefore parents are often punished in their children; or again it is for a remedy intended for the spiritual welfare of the person who suffers these penalties, to wit that he may thus be turned away from his sins, or lest he take pride in his virtues, and that he may be crowned for his patience.
Reply to Objection 5. Death may be considered in two ways. First, as an evil of human nature, and thus it is not of God, but is a defect befalling man through his fault. Secondly, as having an aspect of good, namely as being a just punishment, and thus it is from God. Wherefore Augustine says (Retract. i, 21) that God is not the author of death, except in so far as it is a punishment.
Reply to Objection 6. As Augustine says (De Civ. Dei xiii, 5), "just as the wicked abuse not only evil but also good things, so do the righteous make good use not only of good but also of evil things. Hence it is that both evil men make evil use of the law, though the law is good, while good men die well, although death is an evil." Wherefore inasmuch as holy men make good use of death, their death is to them meritorious.
Reply to Objection 7. Death may be considered in two ways. First, as the privation of life, and thus death cannot be felt, since it is the privation of sense and life. On this way it involves not pain of sense but pain of loss. Secondly, it may be considered as denoting the corruption which ends in the aforesaid privation. Now we may speak of corruption even as of generation in two ways: in one way as being the term of alteration, and thus in the first instant in which life departs, death is said to be present. On this way also death has no pain of sense. In another way corruption may be taken as including the previous alteration: thus a person is said to die, when he is in motion towards death; just as a thing is said to be engendered, while in motion towards the state of having been engendered: and thus death may be painful.
Reply to Objection 8. According to Augustine (Gen. ad lit. [De Pecc. Mer. et Rem. i, 16. Cf. Gen. ad lit. ii. 32), "although our first parents lived thereafter many years, they began to die on the day when they heard the death-decree, condemning them to decline to old age."
Source: Summa Theologica (New Advent)