Arg. 1. The son shall not bear the iniquity of his father (Ezech. xviii, 20).
Reply 1. There is a difference between what affects one individual and what affects the nature of a whole species: for by partaking in the species many men are as one man, as Porphyry says. The sin then that belongs to one individual is not imputable to another individual, unless he sins too, because the one is personally distinct from the other. But any sin touching the specific nature itself may without difficulty be propagated from one to another, as the specific nature is imparted by one to others [by generation].* Since sin is an evil of rational nature, and evil is a privation of good, we must consider of what good the privation is, in order to decide whether the sin in question belongs to our common nature, or is the particular sin of a private individual. The actual sins then, that are commonly committed by men, take away some good from the person of the sinner, such as grace and the due order of the parts of his soul: hence they are personal, and not imputable to a second party beyond the one person of the sinner. But the first sin of the first man not only robbed the sinner of his private and personal good, namely, grace and the due order of his soul, but also took away a good that belonged to the common nature of mankind. According to the original constitution of this nature, the lower powers were perfectly subject to reason, reason to God, and the body to the soul, God supplying by grace what was wanting to this perfection by nature. This benefit, which by some is called 'original justice,'* was conferred on the first man in such sort that it should be propagated by him to posterity along with human nature. But when by the sin of the first man reason withdrew from its subjection to God, the consequence was a loss of the perfect subjection of the lower powers to reason, and of the body to the soul, -- and that not only in the first sinner, but the same common defect has come down to posterity, to whom original justice would otherwise have descended. Thus then the sin of the first man, from whom, according to the doctrine of faith, all other men are descended, was at once a personal sin, inasmuch as it deprived that first man of his own private good, and also a sin of nature (peccatum naturale), inasmuch as it took away from that man, and consequently from his posterity, a benefit conferred upon the whole of human nature.* This defect, entailed upon other men by their first parent, has in those other men the character of a fault, inasmuch as all men are counted one man hy participation in a common nature. This sin is voluntary by the will of our first parent, as the action of the hand has the character of a fault from the will of the prime mover, reason. In a sin of nature different men are counted parts of a common nature, like the different parts of one man in a personal sin.*
Arg. 5. What is natural is no sin, as it is not the mole's fault for being blind.
Reply 5. The defects above mentioned are transmitted by natural origin, inasmuch as nature is destitute of the aid of grace, which had been conferred upon nature in our first parent, and was meant to pass from him to posterity along with nature; and, inasmuch as this destitution has arisen from a voluntary sin, the defect so consequent comes to bear the character of a fault. Thus these defects are at once culpable, as referred to their first principle, which is the sin of Adam; and natural, as referred to a nature now destitute [of original justice].
Arg. 6. A defect in a work of nature happens only through defect of some natural principle.
Reply 6. There is a defect of principle, namely, of the gratuitous gift bestowed on human nature in its first creation; which gift was in a manner 'natural,' not that it was caused by the principles of nature, but because it was given to man to be propagated along with his nature.
Arg. 9. The good of nature is not taken away by sin: hence even in devils their natural excellences remain. Therefore the origin of human generation, which is an act of nature, cannot have been vitiated by sin.
Reply 9. By sin there is not taken away from man the good of nature which belongs to his natural species, but a good of nature which was superadded by grace.*
10. The gift, not belonging to the essence of the species, was nevertheless bestowed by God gratuitously on the first man, that from him it might pass to the entire species: in like manner the sin, which is the privation of that gift, passes to the entire species.
11. Though by the sacraments of grace one is so cleansed from original sin that it is not imputed to him as a fault, -- and this is what is meant by saying that he is personally delivered from that sin, -- yet he is not altogether healed;* and therefore by the act of nature [i.e., of generation] original sin is transmitted to his posterity. Thus then in the human procreant, considered as a person, there is no original sin; and there may very well be no actual sin in the act of procreation: still, inasmuch as the procreant is a natural principle of procreation, the infection of original sin, as regards the nature, remains in him and in his procreative act.*
4.50 : That Original Sin is transmitted from our First Parent to his Posterity
4.56 : Of the Need of Sacraments