Objection 1. It would seem that man can wish and do good without grace. For that is in man's power, whereof he is master. Now man is master of his acts, and especially of his willing, as stated above (1, 1; 13, 6). Hence man, of himself, can wish and do good without the help of grace.
Objection 2. Further, man has more power over what is according to his nature than over what is beyond his nature. Now sin is against his nature, as Damascene says (De Fide Orth. ii, 30); whereas deeds of virtue are according to his nature, as stated above (Question 71, Article 1). Therefore since man can sin of himself he can wish and do good.
Objection 3. Further, the understanding's good is truth, as the Philosopher says (Ethic. vi, 2). Now the intellect can of itself know truth, even as every other thing can work its own operation of itself. Therefore, much more can man, of himself, do and wish good.
On the contrary, The Apostle says (Romans 9:16): "It is not of him that willeth," namely, to will, "nor of him that runneth," namely to run, "but of God that showeth mercy." And Augustine says (De Corrept. et Gratia ii) that "without grace men do nothing good when they either think or wish or love or act."
I answer that, Man's nature may be looked at in two ways: first, in its integrity, as it was in our first parent before sin; secondly, as it is corrupted in us after the sin of our first parent. Now in both states human nature needs the help of God as First Mover, to do or wish any good whatsoever, as stated above (Article 1). But in the state of integrity, as regards the sufficiency of the operative power, man by his natural endowments could wish and do the good proportionate to his nature, such as the good of acquired virtue; but not surpassing good, as the good of infused virtue. But 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.
Reply to Objection 1. Man is master of his acts and of his willing or not willing, because of his deliberate reason, which can be bent to one side or another. And although he is master of his deliberating or not deliberating, yet this can only be by a previous deliberation; and since it cannot go on to infinity, we must come at length to this, that man's free-will is moved by an extrinsic principle, which is above the human mind, to wit by God, as the Philosopher proves in the chapter "On Good Fortune" (Ethic. Eudem. vii). Hence the mind of man still unweakened is not so much master of its act that it does not need to be moved by God; and much more the free-will of man weakened by sin, whereby it is hindered from good by the corruption of the nature.
Reply to Objection 2. To sin is nothing else than to fail in the good which belongs to any being according to its nature. Now as every created thing has its being from another, and, considered in itself, is nothing, so does it need to be preserved by another in the good which pertains to its nature. For it can of itself fail in good, even as of itself it can fall into non-existence, unless it is upheld by God.
Reply to Objection 3. Man cannot even know truth without Divine help, as stated above (Article 1). And yet human nature is more corrupt by sin in regard to the desire for good, than in regard to the knowledge of truth.
Source: Summa Theologica (New Advent)