Rhonda: As long as the cost is not too great, you should use your time, energy, or money to help others. People who are active participants in charitable causes have richer lives than miserly hermits, however prosperous the hermits may be. Brad: You should ignore the problems of complete strangers and focus your generosity on your immediate relatives and close friends, since these are the people who will remember your sacrifices and return the kindness when you yourself need help.

Which one of the following principles, if valid, would most help to justify both Rhonda’s and Brad’s arguments?