 Clarke, Hume’s central rationalist opponent, appeals to reason to explain almost every aspect of morality. He believes that there are demonstrable moral relations of fitness and unfitness that we discover a priori by means of reason alone. Gratitude, for example, is a fitting or suitable response to kindness, while ingratitude is an unfitting or unsuitable response. He believes that the rational intuition that an action is fitting has the power both to obligate us and to move us. To act morally is to act rationally.