On the contrary, (1) God made in man whatever is natural to him.  Now “God made not death” (Wis. 1:13).  Therefore death is not natural to man.

(2) Further, that which is natural cannot be called either a punishment or an evil:  since what is natural to a thing is suitable to it.  But death and such like defects are the punishment of original sin, as stated above (A. 5).  Therefore they are not natural to man.

(3) Further, matter is proportionate to form, and everything to its end.  Now man’s end is everlasting happiness, as stated above (Q. 2, A. 7; Q. 5, AA. 3, 4):  and the form of the human body is the rational soul, as was proved in the First Part (Q. 75, A. 6).  Therefore the human body is naturally incorruptible.

I answer that, We may speak of any corruptible thing in two ways; first, in respect of its universal nature, secondly, as regards its particular nature.  A thing’s particular nature is its own power of action and self-preservation.  And in respect of this nature, every corruption and defect is contrary to nature, as stated in De Coelo ii, text. 37, since this power tends to the being and preservation of the thing to which it belongs.

On the other hand, the universal nature is an active force in some universal principle of nature, for instance in some heavenly body; or again belonging to some superior substance, in which sense God is said by some to be “the Nature Who makes nature.”  This force intends the good and the preservation of the universe, for which alternate generation and corruption in things are requisite:  and in this respect corruption and defect in things are natural, not indeed as regards the inclination of the form which is the principle of being and perfection, but as regards the inclination of matter which is allotted proportionately to its particular form according to the discretion of the universal agent.  And although every form intends perpetual being as far as it can, yet no form of a corruptible being can achieve its own perpetuity, except the rational soul; for the reason that the latter is not entirely subject to matter, as other forms are; indeed it has an immaterial operation of its own, as stated in the First Part (Q. 75, A. 2).  Consequently as regards his form, incorruption is more natural to man than to other corruptible things.  But since that very form has a matter composed of contraries, from the inclination of that matter there results corruptibility in the whole.  In this respect man is naturally corruptible as regards the nature of his matter left to itself, but not as regards the nature of his form.