In the third Critique, Kant claims that beauty is the symbol of morality and that the consideration of this relation is a duty. This paper declares Kant’s argument to be twofold: firstly, experience of beauty strengthens our moral feeling. Secondly, in judging the beautiful, we assume some indeterminate purpose underlying nature, based on which we can conceive of nature as cooperative with our practical pursuit. Hence, for the sake of moral cultivation and moral motivation, it is our duty to regard beauty as the special symbol of morality.