On the contrary, The same is the subject of a virtue and of the vice or sin contrary to that virtue.  But the flesh cannot be the subject of virtue:  for the Apostle says (Rom. 7:18):  “I know that there dwelleth not in me, that is to say, in my flesh, that which is good.”  Therefore the flesh cannot be the subject of original sin, but only the soul.

I answer that, One thing can be in another in two ways.  First, as in its cause, either principal, or instrumental; secondly, as in its subject.  Accordingly the original sin of all men was in Adam indeed, as in its principal cause, according to the words of the Apostle (Rom. 5:12):  “In whom all have sinned”:  whereas it is in the bodily semen, as in its instrumental cause, since it is by the active power of the semen that original sin together with human nature is transmitted to the child.  But original sin can nowise be in the flesh as its subject, but only in the soul.

The reason for this is that, as stated above (Q. 81, A. 1), original sin is transmitted from the will of our first parent to this posterity by a certain movement of generation, in the same way as actual sin is transmitted from any man’s will to his other parts.  Now in this transmission it is to be observed, that whatever accrues from the motion of the will consenting to sin, to any part of man that can in any way share in that guilt, either as its subject or as its instrument, has the character of sin.  Thus from the will consenting to gluttony, concupiscence of food accrues to the concupiscible faculty, and partaking of food accrues to the hand and the mouth, which, in so far as they are moved by the will to sin, are the instruments of sin.  But that further action is evoked in the nutritive power and the internal members, which have no natural aptitude for being moved by the will, does not bear the character of guilt.

Accordingly, since the soul can be the subject of guilt, while the flesh, of itself, cannot be the subject of guilt; whatever accrues to the soul from the corruption of the first sin, has the character of guilt, while whatever accrues to the flesh, has the character, not of guilt but of punishment:  so that, therefore, the soul is the subject of original sin, and not the flesh.

Reply Obj. 1:  As Augustine says (Retract. i, 27) [Cf.  QQ. lxxxiii, qu. 66], the Apostle is speaking, in that passage, of man already redeemed, who is delivered from guilt, but is still liable to punishment, by reason of which sin is stated to dwell “in the flesh.”  Consequently it follows that the flesh is the subject, not of guilt, but of punishment.

Reply Obj. 2:  Original sin is caused by the semen as instrumental cause.  Now there is no need for anything to be more in the instrumental cause than in the effect; but only in the principal cause:  and, in this way, original sin was in Adam more fully, since in him it had the nature of actual sin.

Reply Obj. 3:  The soul of any individual man was in Adam, in respect of his seminal power, not indeed as in its effective principle, but as in a dispositive principle:  because the bodily semen, which is transmitted from Adam, does not of its own power produce the rational soul, but disposes the matter for it.