Agnes Callard in the Boston Review:
Suppose that you are angry on Tuesday because I stole from you on Monday. Suppose that on Wednesday I return what I stole; I compensate you for any disadvantage occasioned by your not having had it for two days; I offer additional gifts to show my good will; I apologize for my theft as a moment of weakness; and, finally, I promise never to do it again. Suppose, in addition, that you believe my apology is sincere and that I will keep my promise.
Could it be rational for you to be just as angry on Thursday as you were on Tuesday? Moreover, could it be rational for you to conceive of a plan to steal from me in turn? And what if you don’t stop at one theft: could it be rational for you to go on to steal from me again, and again, and again?
Though your initial anger at me might have been reasonable, we tend to view a policy of unending disproportionate revenge as paradigmatically irrational. Eventually we should move on, we are told, or let it go, or transmute our desire for revenge into a healthier or more respectable feeling. This idea has given rise to a debate among academic philosophers about the value of anger. Should we valorize it in terms of the righteous indignation of that initial response? Or should we vilify it in terms of the grudge-bearing vengeance of the unending one?