The reason of the difference is, because bonitas physica, or relativa est congruentia naturae quaedem, saith Scalliger; and because two natures may be contrary one to another, therefore the good which is congruous to the one may be contrary to the good which is congruous to the other; but bonum virtutis, saith Aquinas non accipitur nisi per convenientiam ad aliquid unum, scilicet rationem; so that it is impossible for one moral good to be opponed to another.