Hodge:

16. What distinction is intended by the theological terms, natural and moral ability? By natural ability was intended the possession, on the part of every responsible moral agent, whether holy or unholy, of all the natural faculties, as reason, conscience, free will, requisite to enable him to obey God s law. If any of these were absent, the agent would not be responsible.†

By moral ability was intended that inherent moral condition of these faculties, that righteous disposition of heart, requisite to the performance of duty.

Although these terms have been often used by orthodox writers in a sense which to them expressed the truth, yet they have often been abused, and are not desirable. It is evidently an abuse of the word to say that sinners are naturally able, but morally unable, to obey the law; for that can be no ability which leaves the sinner, as the Scriptures declare, utterly unable either to think, feel, or act aright. Besides, the word “natural,” in the phrase “natural ability,” is used in an unusual sense, as opposite to moral; while in the usual sense of that word it is declared in Scripture that man is by nature, i.e., naturally, a child of wrath.

[† Edwards on the Will, part L, sect. 4.]

A.A. Hodge, Outlines of Theology (London: T. Nelson and Sons, 1877), 272. [Footnote included.]