(A sailor who had already depleted his supplies, leaving him with no additional hardtack of his own to eat today, would be in no position to repay borrowed biscuits either.)
That was bad news for thrifty seafarers.
But worse scenarios were possible.
If the sailors had washed ashore with perishable figs rather than imperishable hardtack, the rate of interest would have been steeply negative.
"There is no absolutely necessary reason inherent in the nature of man or things why the rate of interest in terms of any commodity standard should be positive rather than negative," Fisher concluded.
Two years ago, when the Bank of Japan(BoJ) began charging financial institutions for adding to their reserves at the central bank,
its negative-rate policy was harshly criticised for unsettling thrifty households, jeopardising bank profitability and killing growth with "monetary voodoo".
Behind this fear and criticism was perhaps agut conviction that negative rates upended the natural order of things.
Why should people pay to save money they had already earned?
Earlier cuts below zero in Switzerland, Denmark, Sweden and the euro area were scarcely more popular.
But these monetary innovations would have struck some earlier economic thinkers as entirely natural.
Indeed, "The Natural Economic Order" was the title that Silvio Gesell gave to his 1916 treatise in favour of negative interest rates on money.
In it, he span his own shipwreck parable, in which a lone Robinson Crusoe tries to save three years' worth of provisions to tide him over while he devotes his energies to digging a canal.