"IN the state of corrupt nature, man falls short of what he could do by his nature, so that he is unable to fulfil it by his own natural powers. Yet because human nature is not altogether corrupted by sin, so as to be shorn of every natural good, even in the state of corrupted nature it can, by virtue of its natural endowments, work some particular good, as to build dwellings, plant vineyards, and the like; yet it cannot do all the good natural to it, so as to fall short in nothing; just as a sick man can of himself make some movements, yet he cannot be perfectly moved with the movements of one in health, unless by the help of medicine he be cured.
"And thus in the state of perfect nature man needs a gratuitous strength superadded to natural strength for one reason, viz. in order to do and wish supernatural good; but for two reasons, in the state of corrupt nature, viz. in order to be healed, and furthermore in order to carry out works of supernatural virtue, which are meritorious. Beyond this, in both states man needs the Divine help, that he may be moved to act well."
~St. Thomas Aquinas: Summa Theologica, I-II, q. 109, a. 2.
