CNBC reports that President Trump announced Iran has assured the United States there will be no tolls, insurance costs, or charges of any kind on vessels transiting the Strait of Hormuz. That is a fine-sounding declaration, and any policy that keeps a commercial waterway open and free does the work of civilization. Roughly a fifth of the world's oil moves through that narrow passage — inference, not recollection, but a widely attested one — and a toll on it is, in practical effect, a tax levied on every household that burns fuel or buys anything moved by ship.

I spent years thinking about passages and the costs imposed on them. As postmaster, I knew that a road closed or a rate arbitrarily raised does not merely inconvenience the traveler; it suppresses the very commerce and exchange of ideas that knit a republic together. A strait is a postal road writ large upon the ocean.

Yet here is the question I would press, as I pressed it about every instrument of paper credit in my own time: what disciplines the promise? A pledge is paper. Paper money, well-secured, is a useful instrument; paper money badly secured is a slow theft. The same logic applies to a diplomatic assurance. What treaty, what inspection, what mutual interest makes the assurance redeemable? A printer learns early that the value of a note is only as good as the press and the reserves behind it.

I do not say the assurance is worthless — only that its value must be tested against the mechanism of enforcement, not merely the warmth of the announcement. Merchants who insure cargoes through that strait will price the risk themselves, and their premiums will tell you, in cold arithmetic, how much credit the markets extend to the guarantee. Watch the insurance rate; it is the honest ledger when the diplomats' ledger may not be.

My counsel to any working person whose livelihood touches goods that cross that water: welcome the news, but do not yet let it alter your reserves or your contracts. Hold a little back until the promise has been kept through a season or two. As I once observed in another context — an ounce of prevention is worth a pound of cure — and in commerce, that ounce is called a contingency fund. Build one.