Philosophers distinguish between moral duties and moral virtues.

Moral duties must be performed and not to perform them is wrong. Since moral duties must be performed, they must be capable of being performed. In contrast, there is no obligation to do virtuous acts, even though the need to do them may be compelling. For example, if one were a non-swimmer, to seek to rescue a child drowning in deep, shark-infested waters at great personal risk would be to do a virtuous act but not to perform a moral duty, and philosophers generally hold that it would not be wrong not to attempt the rescue. On the other hand, to fail to rescue a child (at no personal risk to oneself) who was drowning in shallow water is probably not to perform a moral duty. What matters are the circumstances of each case – in this particular example, the circumstances concern the risk to the rescuer and whether the rescuer was likely to do the act successfully.

There are three reasons why to forgive is not a moral duty. The first is that it is not always morally right to forgive, and it cannot be a moral duty to do something that is wrong. The second is that sometimes it is not possible to forgive, and it cannot be a moral duty to do the impossible.