Aquinas on the difference between a disposition and a nature

"the vice of a thing seems to consist in its not being disposed in a way befitting its nature."

     Thomas Aquinas, Summa theologiae I-II.71.1.Resp. ("vitium . . . uniuscuiusque rei esse videtur quod non sit disposita secundum quod convenit suae naturae"), all italics mine.  FEDP here.