Last September, after Robert Murphy and Lord Keynes wrote about the Sraffa-Hayek debate of 1932 about the natural rate of interest, I wrote a post about that controversy in which I took an intermediate position defending Hayek against Sraffa’s charge that his use of the natural-rate concept was incoherent, while observing as well that the natural rate of interest in nominal terms is not unique, because any real intertemporal equilibrium is consistent with any choice of price level and any rate of inflation. The condition for a real intertemporal equilibrium with money is simply that the level and rate of change of prices be foreseen correctly. In such an equilibrium, own rates could differ, but by no more than necessary to compensate for different real service flows and different costs of storage associated with different assets, inasmuch as the expected net real return from holding every asset must be equal in equilibrium. But while expected real returns from holding assets must be equal, that unique real return is consistent with any nominal return reflecting any arbitrary rate of price change. It is not by choosing a particular nominal rate of interest — a rate that equals the natural rate — that the monetary authority brings about intertemporal equilibrium. Rather, it is the consistency between whatever nominal interest rate the monetary authority has chosen and the expectations by economic agents of future prices that is the necessary and sufficient condition for intertemporal equilibrium. Any nominal interest rate can become the natural rate if it is supported by an equilibrium set of price expectations. Hayek almost, but not quite, understood this point. His incomplete understanding seems to have prevented him from responding effectively to Sraffa’s charge that his concept of a natural rate of interest was incoherent based on the potential existence of many different own rates of interest in a barter equilibrium.

As a result of last September’s post about Sraffa and Hayek, my colleague Paul Zimmerman and I wrote a paper about the Sraffa-Hayek debate and Keynes’s role in the debate and his later discussion of own rates in chapter 17 of the *General Theory*. I gave a talk about this paper at Brock University in St. Catherines, Ontario on Sunday at the annual meeting of the History of Economics Society. At some point in the near future, I hope the paper will be ready to circulate on the internet and to submit for publication. When it is I will provide a link to it on the blog. So it was an interesting coincidence that two days after the conference, the Sraffa-Hayek debate about the natural rate and about own rates was the subject of renewed interest in the blogosphere.

The latest round was started by Andrew Laiton who wrote about multiple own rates of interest. Laiton apparently thinks that there could be multiple real own rates, but seems to me to overlook the market forces that tend to equalize own rates, market forces wonderfully described by Keynes in chapter 17. Nick Rowe followed up with a post in which he seems to accept that real own rates could differ across commodities, but doesn’t think that that matters. All that matters is that the monetary authority choose a particular own rate and sets its nominal rate to match the chosen own rate. (Daniel Kuehn agrees with Nick here.)

Nick is right that there is no natural rate that can be defined apart from a particular choice of a nominal price path for at least one commodity over time. But in an economy with* n* commodities and* t* time periods, there are *nt* possible choices (actually many more possible choices if we take into account all possible baskets of commodities and all possible rates of price change). The job of the monetary authority is to pin down a path of nominal prices. Given that nominal choice, the natural rate consistent with intertemporal equilibrium would find expression in a particular nominal term structure of interest rates consistent with the equilibrium price expectations of agents. Hayek himself proposed constant NGDP as a possible monetary rule. What Hayek failed to see is that it was the choice of a particular value or time path of nominal GDP that would determine a particular nominal value of the natural rate, not, as Hayek believed, that by choosing a nominal interest rate equal to the natural rate, the monetary authority would ensure that NGDP remained constant over time.