I was recently questioned about the distinction between natural and moral inability. Here's my reply:
If we are to affirm that man is in fact responsible, then he must be just that: response-able. One of the reasons that hypers like to speak of man being "accountable" rather than "responsible" is because they overstate the T in TULIP. They deny any sense of ability in man to believe. What then do Calvinists mean when they speak of "Total Inability" if not that man is without any sense of ability to believe whatsoever?
I think it's better to speak of Total Moral Inability. It's not that man lacks the faculties with which to respond to God in repentance and faith, but that he lacks the desire or want to do so. Man's problem is not in their will power, so to speak, but in their WON'T power.
Fallen man is entirely corrupted by sin. It pollutes their entire persons: mind, body and will. Even though man is fallen and dead in trespasses and sins, they still have the image of God, which AT LEAST consists in their being rational moral agents. Sinners cannot complain that they have not the faculties with which to respond to God. They are constituted by God with minds to think and wills to act. This is what I mean when I speak of "natural ability."
When I speak of the "moral inability" of those still dead in trespasses and sin, I am saying that they're so fixed and determined in their stubborness that they do not even desire to turn to God in truth. The warped affections of their hearts (moral impulses) causes them to perpetually use their God-given intellectual and voluntary instruments (natural abilities or capacities) to play nothing but a cacophonous noise. They are content being in such a state of deadness until the Holy Spirit grants them a new heart with new affections, which creates new moral impulses to use their members (mind, body, wills etc.) to the glory of God.
I am distinguishing between man's constitutional (or natural) abilities and his moral abilities, but I don't want to suggest that we can separate them. They are, admittedly, interrelated. However, what the bible is constantly underlining when it describes our naturally enslaved state is our moral inability, and not our lack of constitutional capacities.
When God regenerates us, he doesn't give us new personalities or new faculties. It's just the case that our faculties are renewed even as our hearts are renewed. When quickened by the Spirit, our wills change because our spiritual affections have changed. The regenerate are now motivated by a love to God and the love of our neighbor. This is now our dominating moral impulse, even though we still struggle with remaining pollution (the flesh principle). Consequently, our minds are no longer "set on the flesh" as they once were.
Does that help you understand the distinction I am trying to make? "Ought" does imply "can" in some sense (in the sense that we possess the faculties with which to respond to God's commands), but "ought" does not imply "can" in another sense (in the sense that our mind and/or wills are morally neutral, or without a dominant moral impulse).
I then provided this quote by Stephen Charnock that relates to the same subject:
"(2.) It doth not disparage his wisdom to command that to man which he knows man will not do without his grace, and so make promises to man upon the doing it. If man indeed had not a faculty naturally fitted for the object, it might entrench upon God's wisdom to make commands and promises to such a creature as it would be to command a beast to speak. But man hath a faculty to understand and will, which makes him a man; and there is a disposition in the understanding and will which consists in an inclination determined to good or evil, which makes us not to be men, but good or bad men, whereby we are distinguished from one another as by reason and will we are from plants and beasts. Now the commands and exhortations are suitable to our nature, and respect not our reason as good or bad, but simply as reason. These commands presuppose in us a faculty of understanding and will, and a suitableness between the command and the faculty of a reasonable creature. This is the reason why God hath given to us his law and gospel, his commands, not because we are good or bad men, but because we are men endued with reason, which other creatures want, and therefore are not capable of government by a command. Our blessed Lord and Saviour did not exhort infants, though he blessed them, because they were not arrived to the use of reason; yet he exhorted the Jews, many of whose wills he knew were not determined to good, and whom he told that they would die in their sins. And though God had told them, Jer. xiii., that they could no more change themselves than an Ethiopian could his skin, yet he expostulates with them why they 'would not be made clean:' verse 27, 'O Jerusalem, wilt thou not be made clean? when shall it once be?' Because, though they had an ill disposition in their judgment, yet their judgment remained, whereby to discern of exhortations if they would. To present a concert of music to a deaf man that cannot hear the greatest sound were absurd, because sounds are the object of hearing; but commands and exhortations are the object, not of this or that good constitution of reason, but of reason itself."
Stephen Charnock, "A Discourse of the Efficient of Regeneration" in The Works of Stephen Charnock (Carlisle, Penn.: Banner of Truth, 1986), 3:227-228.
For those interested, Jonathan Edwards makes the distinction in his work on the Freedom of the Will.