 The most plausible candidates for obligations of gratitude are instances of grateful behavior (see  section 3.4).  Ordinary language seems to speak in favor of some of these behaviors being obligatory. The metaphor of debts of gratitude, which a beneficiary pays to a benefactor by returning a favor, dates back centuries, at least in the Anglophone linguistic tradition (Brandt 1964: 386); and philosophers and writers throughout history have condemned acts and omissions of ingratitude as among the most vicious of wrongs. Hume (1738 [2007]: III.I.I), for instance, wrote that “Of all crimes that human creatures are capable of committing, the most horrid and unnatural is ingratitude…” Kant wrote that ingratitude was (along with envy and malice) “the essence of vileness and wickedness” (Kant 1775–1780 [1981]). And Seneca (On Benefits) ranked ingrates below thieves, rapists and adulterers.