Thomas Aquinas, Compendium of Theology

But there remains a more pressing question: whether the privation of original justice can have the nature of sin in those who descend from the first parent. The notion of sin seems to require, as we said above, that the evil known as culpable should be in the power of him to whom it is imputed as fault. No one is blamed for that which is beyond his power to do or not to do. But it is not in the power of the person begotten, to be born with original justice or without it. Hence we might be inclined to judge that such a privation cannot have the character of sin.

This question is easily solved if we but distinguish between person and nature. As there are many members in one person, so there are many persons in one human nature. Thus, by sharing in the same species, many men may be thought of as one man, as Porphyry remarks. In this connection we should note that in the sin of one man different sins are committed by different members. Nor does the notion of sin require that the various sins be voluntary by the wills of the members whereby they are committed, for it is enough that they be voluntary by the will of that which is most excellent in man, that is, his intellectual part. For the hand cannot but strike and the foot cannot help walking, when the will so commands.

In this way, then, the privation of original justice is a sin of nature, in the sense that it has its origin in the inordinate will of the first principle in human nature, namely, of the first parent. Thus it is voluntary with respect to nature, that is, by the will of the first principle of nature. And so it is transmitted to all who receive human nature from him, for they are all, as it were, his members.

This is why it is called original sin, for it is transferred from the first parent to his descendants by their origin from him. Other sins, that is, actual sins, pertain immediately to the person who commits them; this sin directly touches nature. The first parent infected nature by his sin, and nature thus contaminated thereupon infects the persons of the children who receive their nature from the first parent.