I don't think you need a separate rule, having a prior covers it.

What Mungers statement makes me think of is an alternative approach to weakness; I think the default approach for many people is to "improve themselves". In many cases this might just not be worth the time; looking for an imperfect "opposite virtue" might be workable. E.g. if life demands you to rise early instead of trying to become an "early riser" get an alarm that works and move on.