Bills of Exchange "can be" issued against thin air. They can but should not. Such "dry" (financial) bills were never accepted in free circulation. The ancient church even strictly forbade their issue under usury rules. Merchants strictly only accepted commercial bills issued "against value". This means they were created as medium of exchange ("money") as the 1st leg against a sale of B2B goods. (Akin to Bitcoiners "proof-of-work"). Bills issued against goods CANCEL OUT when used to purchase final goods in the 2nd leg of real exchange. Bills were the indispensable enablers of exchange. And will be in Bitcoin. Study Bitcredit Protocol !