Generally treated as a duty, but an interesting one in that the duty is not to act but to feel a certain way. Although the concept involves the idea of repayment, gratitude is not simply the just repayment of debt, for the benefactor cannot demand gratitude in the way a creditor can demand repayment. Also we should feel gratitude for things that we cannot in any meaningful sense repay, such as life (or, Aristotle argues, a training in philosophy). What is required is a feeling of love or honour for the benefactor, and deficiency in this feeling is a failing so severe as to be, in Hume’s words, ‘of all crimes that human beings are capable of committing, the most horrid and unnatural’.