Natural and moral ability were clearly defined and distinguished by Jonathan Edwards in his work, The Freedom of the Will. Andrew Fuller then popularized Edwards's views among Particular Baptists, providing them with a theological escape from hyper-Calvinism. Interestingly though, today in my seminar on the history of doctrinal anthropology, a friend of mine challenged the validity and helpfulness of the distinction. He pointed out that a number of solid Reformed theologians have objected to it for very specific reasons. The objectors include L. Berkhof, W.G.T. Shedd, C. Hodge, and A.A. Hodge. Dr. Nettles added to the list by pointing out that both R.L. Dabney and Ian Murray have also taken issue with Edwards's arguments.
A.A. Hodge urged caution in distinguishing between moral and natural ability for a number of reasons. They include:
1. The terms "natural ability" and "moral ability" are not Scriptural terms, and the Bible only teaches that men are unable, never that they are able.
2. The terms have never been adopted in the creed statements of the Reformed churches.
3. The terminology is essentially ambiguous. "The language assures [a person] that he is able in a certain sense, when it is only true that he possesses some of the essential prerequisites of ability. Ability begins only after all its essential conditions are present."
4. The word "natural" is not the antithesis of "moral," since there is such a thing as a "moral nature."
5. The language does not accurately express the distinction intended: "The inability is moral and is not either physical or constitutional. It has its ground not in the want of any faculty, but in the corrupt moral state of the faculties."
See A.A. Hodge, Outlines of Theology (reprint, Carlisle, PA: Banner, 1991), 341-342.