The rules for practicing virtue (exercitiorum virtutis) aim at a frame of mind that is both valiant and cheerful in fulfilling its duties (animus strenuus et hilaris). For, virtue not only has to muster all its forces to overcome the obstacles it must contend with; it also involves sacrificing many of the joys of life, the loss of which can sometimes make one's mind gloomy and sullen....With regard to the principle of a vigorous, spirited, and valiant practice of virtue, the cultivation of virtue, that is, moral ascetics, takes as its motto the Stoic saying: accustom yourself to put up with the misfortunes of life that may happen and to do without its superfluous pleasures (assuesce incommodis et descuesce commoditatibus vitae).... Something must be added to it, something which, though it is only moral, affords an agreeable enjoyment to life. This is the ever-cheerful heart, according to the idea of the virtuous Epicurus.
[Immanuel Kant, The Metaphysics of Morals 6:484-485, as translated in Immanuel Kant, Practical Philosophy, Gregor, tr., Cambridge University Press (New York: 2008) p. 597.]
