I answer that, We may speak of man in two ways: first, in the state of perfect nature; secondly, in the state of corrupted nature. Now in the state of perfect nature, man, without habitual grace, could avoid sinning either mortally or venially; since to sin is nothing else than to stray from what is according to our nature---and in the state of perfect nature man could avoid this. Nevertheless he could not have done it without God's help to uphold him in good, since if this had been withdrawn, even his nature would have fallen back into nothingness.
But in the state of corrupt nature man needs grace to heal his nature in order that he may entirely abstain from sin. And in the present life this healing is wrought in the mind---the carnal appetite being not yet restored. Hence the Apostle (Rm. 7:25) says in the person of one who is restored: "I myself, with the mind, serve the law of God, but with the flesh, the law of sin." And in this state man can abstain from all mortal sin, which takes its stand in his reason, as stated above (Question [74], Article [5]); but man cannot abstain from all venial sin on account of the corruption of his lower appetite of sensuality. For man can, indeed, repress each of its movements (and hence they are sinful and voluntary), but not all, because whilst he is resisting one, another may arise, and also because the reason is always alert to avoid these movements, as was said above (Question [74], Article [3], ad 2).
So, too, before man's reason, wherein is mortal sin, is restored by justifying grace, he can avoid each mortal sin, and for a time, since it is not necessary that he should be always actually sinning. But it cannot be that he remains for a long time without mortal sin. Hence Gregory says (Super Ezech. Hom. xi) that " a sin not at once taken away by repentance, by its weight drags us down to other sins": and this because, as the lower appetite ought to be subject to the reason, so should the reason be subject to God, and should place in Him the end of its will. Now it is by the end that all human acts ought to be regulated, even as it is by the judgment of the reason that the movements of the lower appetite should be regulated. And thus, even as inordinate movements of the sensitive appetite cannot help occurring since the lower appetite is not subject to reason, so likewise, since man's reason is not entirely subject to God, the consequence is that many disorders occur in the reason. For when man's heart is not so fixed on God as to be unwilling to be parted from Him for the sake of finding any good or avoiding any evil, many things happen for the achieving or avoiding of which a man strays from God and breaks His commandments, and thus sins mortally: especially since, when surprised, a man acts according to his preconceived end and his pre-existing habits, as the Philosopher says (Ethic. iii); although with premeditation of his reason a man may do something outside the order of his preconceived end and the inclination of his habit. But because a man cannot always have this premeditation, it cannot help occurring that he acts in accordance with his will turned aside from God, unless, by grace, he is quickly brought back to the due order.
