Shedd:
1) In the Westminster statement, the disability or inability is connected with the disposition and inclination of the will. Man is “indisposed to all spiritual good, and inclined to all [spiritual] evil.” It follows from this, that the cause and seat of the inability in question is in the action and state of the voluntary faculty. It is moral or willing inability.
Nam servit voluntas peccato, non nolens sed volens. Etenim voluntas non noluntas dicitur. Second Helvetic Confession, IX.
In denominating it “moral” inability, it is not meant that it arises merely from habit, or that it is not “natural” in any sense of the word nature. A man is sometimes said to be morally unable to do a thing, when it is very difficult for him to do it by reason of an acquired habit, but not really impossible. This is not the sense of the word “moral” when applied to the sinner’s inability to holiness. He is really and in the full sense of the word impotent. And the cause of this impotence is not a habit of doing evil which he has formed in his individual life, but a natural disposition which he has inherited from Adam. The term “moral,” therefore, when applied to human inability denotes that it is voluntary, in distinction from created. Man’s impotence to good does not arise from the agency of God in creation, but from the agency of man in apostasy.
Whether, therefore, it can ever be called “natural” inability, will depend upon the meaning given to the term “nature.”
(a) If “nature” means that which is created by God, there is no natural inability to good in fallen man. But if “nature” means “natural disposition,” or “natural inclination,” there is a “natural” inability to good in fallen man.