It is the lack of a limiting principle that has always alarmed critics of Lord Hale’s argument. If a barrel of wine takes on the public interest when it crosses a river and then carries that interest with it, effectively undiminished, everywhere it goes, then the public sphere is effectively infinite, as is the mandate for regulation. In the American context, the king’s command over the commons has been superseded most significantly by the doctrine of “interstate commerce.” Like Hale’s ferry principle, the idea of interstate commerce has proved elastic. One would think that interstate commerce would consist only of those activities that are 1) interstate and 2) commercial, but U.S. law has long made room for regulating activities that might plausibly have some remote effect on interstate commerce. In the infamous case of Wickard v. Filburn, the Court held that Roscoe Filburn, a farmer, was subject to federal regulation of his wheat output, which was limited under New Deal law, even though he was growing wheat for his own use on his own farm rather than trading it across state lines. The central planners of the Roosevelt administration argued that they were pursuing a national interest in managing wheat prices and that Filburn’s farm might have some conceivable effect on them: If he grows his own wheat, he isn’t buying it on the interstate market. The Court agreed, and we still live under that precedent.

From the regulation of public waterways comes the principle that a man may not grow wheat on his own farm for his own use without the king’s permission: The slope is, in fact, slippery.