The previous explanation of natural and moral Necessity may also shed light on the concepts of natural and moral Inability. Someone would be naturally unable to do something when he cannot do it because nature does not allow it or because of some “obstacle that is extrinsic to the Will; either in the Faculty of understanding, constitution of body, or external objects.” However, none of those things constitutes moral Inability, which may be defined simply as a want of inclination.
Some examples of moral Inability are the following. A person “of great honor and chastity may have a moral Inability” to engage in prostitution. A child who is devoted to and truly loves his parents may be morally unable to kill his father. “A very malicious man may be unable to exert benevolent acts to an enemy.” Strength of virtue and a great degree of holiness may result in one’s being morally unable to “love wickedness in general.” On the other hand, the strength of one’s habitual wickedness may make him unable to “love and choose holiness.” Such a person may be “utterly unable to love an infinitely holy Being, or to choose and cleave to him as his chief good.”
Regarding this idea of moral Inability, I think it is useful to draw a distinction between the general/habitual and the particular/occasional. A general/habitual moral Inability is “an Inability in the heart to all exercises or acts of the will of that kind.” Thus, a person “whose heart is habitually void of gratitude” may be unable to show gratitude through his actions. By contrast, a particular/occasional moral Inability is an Inability relating to a particular act at a particular point in time. If it is true that “the Will is always determined by the strongest motive,” it follows that the will is always unable (a particular/occasional Inability) to “act otherwise than it does.” It is not possible for the Will to “go against the motive which has now, all things considered, the greatest advantage to induce it.”
The second of these kinds of moral Inability is commonly referred to simply as “Inability.” because the word’s original meaning related “to some stated defect.” In addition, as we noted before, Inability is most commonly used as a relative term; it relates to a will and endeavor that is “insufficient to bring to pass the thing desired and endeavored.” Whether occasional or habitual, a will and endeavor against present acts of the will cannot even be supposed; “that would be to suppose the Will, as present, to be otherwise than, at present, it is.” There conceivably could be, however, will and endeavor “against future acts of the Will.” There is no contradiction in saying that acts of the Will at one time “may be against the acts of the Will at another time.” One might desire to prevent a future act of the Will, but that desire might be overcome by the strength of a fixed habit. “In this respect, a man may be in miserable slavery and bondage to a strong habit.” On the other hand, it may be much easier to prevent future acts that “are only occasional and transient.” In this way, the moral Inability associated with fixed habits “especially obtains the name of Inability.” So just as the will may, in this sense, resist itself in vain, reason may resist a present act of the Will, although its resistance may be insufficient.
Again, with each kind of moral Inability, the word “Inability” is being used very differently from its original meaning. The word refers to a natural Inability, those cases where it can be supposed a person is inclined to do an act, but is unable. Under the ordinary definition, we cannot say that “a malicious man . . . cannot hold his hand from striking.” Strictly speaking, a man is able to do something “if he has it in his choice, or at his election”; we would not say that a man cannot do a thing “when he can do it if he will.” It is incorrect to say that someone is unable to do a thing (something that depends on an act of the Will) that “would be easily performed if the act of the Will were present.” It would be even worse to say that he is unable to “exert the acts of the Will themselves.” That would be a contradiction; to say “he cannot will, if he does will.” Therefore, we should not attribute non-performance to a lack of power or ability. What is lacking is not ability, but one’s being willing. A person may understand, and have the natural ability and everything else necessary, except a disposition. The only thing missing is a will.