The first three objections argue on the side of the matter; while the other three argue on the side of the form.  Wherefore in order to solve them, we must observe that the form of man which is the rational soul, in respect of its incorruptibility is adapted to its end, which is everlasting happiness:  whereas the human body, which is corruptible, considered in respect of its nature, is, in a way, adapted to its form, and, in another way, it is not.  For we may note a twofold condition in any matter, one which the agent chooses, and another which is not chosen by the agent, and is a natural condition of matter.  Thus, a smith in order to make a knife, chooses a matter both hard and flexible, which can be sharpened so as to be useful for cutting, and in respect of this condition iron is a matter adapted for a knife:  but that iron be breakable and inclined to rust, results from the natural disposition of iron, nor does the workman choose this in the iron, indeed he would do without it if he could:  wherefore this disposition of matter is not adapted to the workman’s intention, nor to the purpose of his art.  In like manner the human body is the matter chosen by nature in respect of its being of a mixed temperament, in order that it may be most suitable as an organ of touch and of the other sensitive and motive powers.  Whereas the fact that it is corruptible is due to a condition of matter, and is not chosen by nature:  indeed nature would choose an incorruptible matter if it could.  But God, to Whom every nature is subject, in forming man supplied the defect of nature, and by the gift of original justice, gave the body a certain incorruptibility, as was stated in the First Part (Q. 97, A. 1).  It is in this sense that it is said that “God made not death,” and that death is the punishment of sin.

This suffices for the Replies to the Objections.
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QUESTION 86

OF THE STAIN OF SIN
(In Two Articles)

We must now consider the stain of sin; under which head there are two points of inquiry: 

(1) Whether an effect of sin is a stain on the soul?

(2) Whether it remains in the soul after the act of sin?
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FIRST ARTICLE [I-II, Q. 86, Art. 1]

Whether Sin Causes a Stain on the Soul?

Objection 1:  It would seem that sin causes no stain on the soul.  For a higher nature cannot be defiled by contact with a lower nature:  hence the sun’s ray is not defiled by contact with tainted bodies, as Augustine says (Contra Quinque Haereses v).  Now the human soul is of a much higher nature than mutable things, to which it turns by sinning.  Therefore it does not contract a stain from them by sinning.

Obj. 2:  Further, sin is chiefly in the will, as stated above (Q. 74, AA. 1, 2).  Now the will is in the reason, as stated in De Anima iii, text. 42.  But the reason or intellect is not stained by considering anything whatever; rather indeed is it perfected thereby.  Therefore neither is the will stained by sin.