(An idea I've been playing with lately, just wanted to get something on virtual paper.)
Currency, dollar bills, are liabilities of the Federal Reserve. Federal debt is a liability of the US Treasury. Leaving aside some deliberate obscurity around the precise legal status of the Fed, both are liabilities of the US government. Of course there are various formal differences, but economically, wouldn't it be simplest to regard them the same?
In other words, while we are taught that there are no close substitutes for money but that government and private debt are substitutes for each other, wouldn't it be better to say that money and government debt are substitutes for each other and both are complements for private debt? More concretely, given lenders' need for liquidity, an increase in their holding of government debt may make them more willing to hold private debt, i.e. under some circumstances, an increase in government debt could put downward rather than upward pressure on interest rates further out the yield curve.
The canonical case is the recent financial crisis. As I've discussed before, there's an argument (which gets at least some support from non-crazies like Perry Mehrling and Brad DeLong) that insufficient federal debt contributed to the crisis, by creating demand for equivalently liquid but higher-return substitutes, thus fueling the financial innovation of the 1990s and 2000s. Of course this dynamic would have increased the availability of credit for private borrowers whose liabilities were (a) seen as implicitly enjoying a federal guarantee and (b) easily securitizable. But for borrowers that didn't meet those criteria, the lack of enough new federal debt may have made banks less willing to lend to them, in exactly the same way a lack of reserves would have in the old days. And even if the restriction of credit to non-securitizable borrowers was not that big in the boom, the lack of federal debt certainly exacerbated the crash.
So far this is just thinking aloud. But a bunch of smart people seem to be heading in this direction. Take for instance Roger Farmer's call for more quantitative easing (via DeLong). Says Farmer, "Even if the Bank of England were to buy the entire UK national debt, this policy would not be inflationary." This is just a dramatic way of making the point that as government debt and money have become closer substitutes, the economic consequences of shifts between them have become smaller. As Farmer says, money no longer occupies a discrete, unique role. Instead, there is a continuum of assets: "At the safe end of the spectrum there is cash. At the risky end there is equity and low grade bonds." And in a rich country like the US or UK, government debt is very close to the money end. Where Farmer is less convincing is his idea that the interchangeability of money and public debt came about all at once, when central banks began paying interest on reserves. Seems to me it was a longer process of institutional evolution.
One implication of this, again, is that a smoothly functioning financial system requires more public debt, indefinitely. (Another reason to agree with Davidson, Galbraith and Skidelsky that austerity tomorrow is no more desirable than austerity today.) But there's a second implication: If money as a discrete category is obsolete, then so is monetary policy as we know it. If Treasuries are as liquid as so-called high-powered money, then monetary policy -- which comes down to injecting and removing liquidity -- must work on the former and not just the latter; but of course the volume of federal debt is orders of magnitude greater than the volume of reserves. Which suggests that quantitative easing may be the only kind of easing there is, from here on out, that is, no more distinction between monetary and fiscal policy.
EDIT: What's the affinity between cranks and money? Everyone knows that discussions of monetary theory bring all the cranks to the yard. But am I the only one who finds that writing about this stuff, makes me feel like a crank?