There’s no such thing as base money anymore
Tim Duy has a great review of why platinum coin seigniorage was a bridge too far for Treasury and the Fed. I think he’s pretty much spot on.
However, with Greg Ip (whose objection Duy cites), I’d take issue with the following:
Ultimately, I don’t believe deficit spending should be directly monetized as I believe that Paul Krugman is correct — at some point in the future, the US economy will hopefully exit the zero bound, and at that point cash and government debt will not longer be perfect substitutes.
Note that there are two distinct claims here, both of which are questionable. Consistent with the “Great Moderation” trend, the so-called “natural rate” of interest may be negative for the indefinite future, unless we do something to alter the underlying causes of that condition. We may be at the zero bound, perhaps with interludes of positiveness during “booms”, for a long time to come.
But maybe not. Maybe we’ll see the light and enact a basic income scheme or negative income tax brackets. Maybe we’ll restore the dark, and engineer new ways of providing fraudulently loose credit. Either sort of change could bring “full employment” interest rates back above zero. Let’s suppose that will happen someday.
What I am fairly sure won’t happen, even if interest rates are positive, is that “cash and government debt will no longer be perfect substitutes.” Cash and (short-term) government debt will continue to be near-perfect substitutes because, I expect, the Fed will continue to pay interest on reserves very close to the Federal Funds rate. (I’d be willing to make a Bryan-Caplan-style bet on that.) This represents a huge change from past practice — prior to 2008, the rate of interest paid on reserves was precisely zero, and the spread between the Federal Funds rate and zero was usually several hundred basis points. I believe that the Fed has moved permanently to a “floor” system (ht Aaron Krowne), under which there will always be substantial excess reserves in the banking system, on which interest will always be paid (while the Federal Funds target rate is positive).
If Ip and I are right, Paul Krugman is wrong to say
It’s true that printing money isn’t at all inflationary under current conditions — that is, with the economy depressed and interest rates up against the zero lower bound. But eventually these conditions will end.
Printing money will always be exactly as inflationary as issuing short-term debt, because short-term government debt and reserves at the Fed will always be near-perfect substitutes. In the relevant sense, we will always be at the zero lower bound. Yes, there will remain an opportunity cost to holding literally printed money — bank notes, platinum coins, whatever — but holders of currency have the right to convert into Fed reserves at will (albeit with the unnecessary intermediation of the quasiprivate banking system), and will only bear that cost when the transactional convenience of dirty paper offsets it. In this brave new world, there is no Fed-created “hot potato”, no commodity the quantity of which is determined by the Fed that private holders seek to shed in order to escape an opportunity cost. It is incoherent to speak, as the market monetarists often do, of “demand for base money” as distinct from “demand for short-term government debt”. What used to be “monetary policy” is necessarily a joint venture of the central bank and the treasury. Both agencies, now and for the indefinite future, emit interchangeable obligations that are in every relevant sense money. 
I’ve no grand ideological point to make here. But I think a lot of debate and commentary on monetary issues hasn’t caught up with the fact that we have permanently entered a brave new world in which there is no opportunity cost to holding money rather than safe short-term debt, whether we are at the zero bound or not.
 Yes, there are small frictions associated with converting T-bills to reserves or cash for use as a medium of exchange. I think they are too small to matter. But suppose I’m wrong. Then nonusability as means of payment would mean a greater opportunity cost for T-bill holders than for reserve holders. That is, printing money outright would be less inflationary than issuing short-term debt! And for now, when Fed reserves pay higher interest rates than short-term Treasury bills, people concerned about inflation should doubly prefer “money printing” to short-term debt issuance! Quantiative easing is currently disinflationary in terms of any mechanical effect via the velocity of near-money, when the Fed purchases short-term debt (although it may be inflationary via some expectations channel, because of the intent that’s communicated). The mechanical effect of QE is less clear when the Fed purchases longer maturity debt, it would depend on how market participants trade-off the yield premium and interest rate risk, as well as on what long-term debt clienteles — pension funds etc. — choose to substitute for the scarcer assets. But it is not at all obvious that “printing money” to purchase even long maturity assets is inflationary when the Fed pays a competitive interest rate on reserves.
Thanks to Kid Dynamite for helping me think through some of these issues in correspondence (though he doesn’t necessarily agree with me on any of it!)