legal nature of "promise to fulfill" natural obligations

Binding to execute an obligation is a kind of contractual obligation, which is submitted to the general terms and conditions of validity of a legal agreement. But the question is to know whether the simple promise of completion a natural obligation could make it enforceable by debtor’s unique will, or creditor’s consent is also necessary.
Doctrinal and jurisdictional interpretations in this matter are quite different from each other. Some consider it as a new legal agreement to fulfill a moral duty. While the others think that it is not, but a simple avowal to confirm a moral obligation. Still some jurists believe that the new agreement is a substitution of a new legal contract for an existing natural obligation.