The second essay of Nietzsche's Genealogy of Morality (GM) offers a naturalistic and developmental account of the emergence of conscience, a faculty uniquely responsive to remembering and honoring obligations. This article attempts to solve an interpretive puzzle that is invited by the second essay's explanation of nonmoral obligation, prior to the capacity to feel guilt. Ostensibly, Nietzsche argues that the conscience and our concept of obligation originated within contractual (“creditor-debtor”) relations, when creditors punished delinquent debtors (GM II:5). However, this interpretation, which I call the contractualist reading, is incoherent and subject to an insoluble bootstrapping problem. I argue instead that Nietzsche provides two accounts of nonmoral obligation in the second essay, and that the conscience originated in the morality of custom to track rule prohibitions (“I will nots” [GM II:3]), which Nietzsche conceives of as involuntary or reciprocal obligations that, unlike contractual debts, do not require the making of promises.