Again, the best thing in any government is to provide for the things governed according to their own mode, for the justice of a regime consists in this. Therefore, as it would be contrary to the rational character of a human regime for men to be prevented by the governor from acting in accord with their own duties—except, perhaps, on occasion, due to the need of the moment—so, too, would it be contrary to the rational character of the divine regime to refuse permission for created things to act according to the mode of their nature. Now, as a result of this fact, that creatures do act in this way, corruption and evil result in things, because, due to the contrariety and incompatibility present in things, one may be a source of corruption for another. Therefore, it does not pertain to divine providence to exclude evil entirely from the things that are governed.
[Thomas Aquinas, Summa Contra Gentiles 3.71.4. 'Due to the need of the moment' seems a little weak to me here; I would have translated 'except, perhaps, sometimes on occasion, because of some necessity'. That is, Aquinas seems to be emphasizing the rareness, not qualifying the necessity.]
