If there is question of supernatural good, -- as faith, hope, charity, or other virtues practised on motives of faith, hope, and charity -- man can do not a single act of such good, still less persevere in it, without the aid of grace. If there is question of natural good, -- as justice, fortitude, temperance, practised on a motive of 'sweet reasonableness,' -- we must further enquire whether the question is asked of man historically considered, as a fallen creature; then we must take St Thomas's answer, Sum. Theol. 1a-2ae, q. 109, art. 8; or of man as the a priori philosopher views him, merely as man. In the latter view this fact still seems discernible, that, the acts of virtue being difficult, and the call for such acts continual, man under such a series of trials is likely to break down at times, if he have no other support than the mere liberty of his will. Aristotle refers him to the support of acquired habits of virtue (Eth. Nic. II-IV), a considerable support indeed, but these habits are difficult to acquire, and Aristotle seems to exaggerate the facility which they afford of well doing, and the security that comes of them against a fall. So even in his pure natural condition man seems to be a feeble creature, that cannot be trusted to walk quite alone, but needs "some aid of divine providence governing him from without," and disposing external circumstances in his favour. The aid and kind provision of environment, making for morality, which would have to be looked for from God by man even in his pure natural condition, has been termed by some theologians 'natural grace.'
Of God and His Creatures: 3.156