Remember that there are only three kinds of things anyone need ever do. (1) Things we ought to do (2) Things we’ve gotto do (3) Things we like doing. I say this because some people seem to spend so much of their time doing things for none of the three reasons, things like reading books they don’t like because other people read them. Things you ought to do are things like doing one’s school work or being nice to people. Things one has got to do are things like dressing and undressing, or household shopping. Things one likes doing — but of course I don’t know what you like. Perhaps you’ll write and tell me one day.
Nearly a decade later, in a letter dated July 18, 1957, Lewis revisit the subject of duty’s false deities with another little girl, Joan:
A perfect man wd. never act from a sense of duty; he’d always want the right thing more than the wrong one. Duty is only a substitute for love (of God and of other people), like a crutch, which is a substitute for a leg. Most of us need the crutch at times; but of course it’s idiotic to use the crutch when our own legs (or own loves, tastes, habits etc) can do the journey on their own!