Grace perfects nature by requiring it to die to itself

     "The real does not appear in the same light in both cases.  The theologian declares that grace perfects nature and does not destroy it; the saint declares that grace requires us to make nature die to itself.  They are both telling the truth.  But it would be a shame to reverse their languages by making use in the speculative order of formulas which are true for the practical order, and vice versa."

     Jacques Maritain, The peasant of the Garonne:  an old layman questions himself about the present time, trans. Michael Cuddihy and Elizabeth Hughes (New York:  Holt, Rinehart and Winston), 44, italics mine.  See also p. 240:

it is of course true that grace perfects nature and does not destroy it, but this means in effect that grace perfects nature by going beyond it, and transforms it (according to the law of all transformation) by making it give up that which, in its own order, and not without reason, it holds most dear.

     Cf. this entry on Aquinas.