Summa contra Gentiles3.160 Chapter CLX. That a man who is in sin cannot avoid sin without grace

The statement that it is in the power of the free-will to offer no obstacle to grace, applies to those in whom the natural power retains its integrity. If however, through some previous disorder, it has turned aside to evil ways, it will not be wholly in its power to place no obstacles to grace. For though man, by his own power, is able to refrain at a certain moment, from a particular sinful act: yet if he be left to himself for long, he will fall into sin, whereby an obstacle to grace is set up. Because when the human mind has turned aside from the path of rectitude, it is clear that it has abandoned the direction to its due end. Consequently that which should stand first in its affections, as its last end, becomes less loved than the thing to which the mind has inordinately turned as though it were its last end. Hence whenever something presents itself that is suitable for an inordinate end, and incompatible with the right end, it will be chosen, unless the mind be brought into right order, so that it places its last end before all: and this is the effect of grace. But as long as a thing is chosen that is incompatible with the last end, an obstacle is opposed to grace which directs us to our end. Wherefore it is evident that, after having sinned, man cannot refrain from all sin, before being restored to the right order by grace.

Again. Once the mind is inclined to something, it is no longer equally disposed towards either of two opposites, but is more disposed to that one to which it is inclined. Now, the mind chooses the thing to which it is more disposed, unless through the reason discussing the matter, it become disinclined thereto from motives of precaution: hence it is chiefly under unforeseen circumstances that a person’s conduct is a sign of his interior disposition. Now, it is not possible for a man’s mind to be continually so wide awake as to deliberate about everything that is to be willed or done. Hence it follows that sometimes the mind chooses the object to which it is inclined, because the inclination remains. And so, if it be inclined to sin, it will not stay long without sinning, through placing an obstacle to grace, unless it be restored to the state of rectitude.

To this also the impulse of the bodily passions conduces; also objects of sensible appetite, and occasions of evil-doing: for by these things man is easily incited to sin, unless he be held in check by a firm adherence to his last end, which is the effect of grace.

Hence we can see the absurdity of the opinion of the Pelagians, who held that man, while in a state of sin, is able to avoid sin without grace. The contrary of this may be gathered from the petition of the Psalm (Ixx. 9): When my strength shall fail, do not Thou forsake me. Moreover our Lord taught us to pray: And lead us not into temptation, hut deliver us from evil.

However, albeit those who are in sin cannot, of their own power, avoid placing an obstacle to grace, as we have proved, unless they be assisted by prevenient grace; nevertheless this is imputed to them as sin, because the defect in question remains in them through their preceding fault: even so a drunken man is not excused from murder committed through his being in a drunken state, which he incurred through his own fault.

Moreover, although a man who is in sin, has it not in his power to avoid sin altogether, yet it is in his power to avoid a particular sin at a given moment, as we have said. Hence, whatever sin he commits, he commits it deliberately. Consequently it is not undeservedly imputed to him as a sin.