The economists’ commonplace that a monetary union demands a fiscal union is only part of a much deeper truth about debt and obligation: debt is rarely sustainable if there is not some sense of communal or collective responsibility. That is the mechanism that reduces the incentives to expropriate the creditor, and makes debt secure and cheap.
“统一货币需要统一财政”，这句经济学家的老生常谈，其实揭示了一条更深刻的关于债务的真理：若没有责任共担之心，便难有可靠之债。只有遵循这个道理，才能打消赖账之心，让债务变得安全而便宜。