We should have had a bigger argument about it 30 years ago, which would have sorted it all out. But we didn't. Maybe because we spent all our time arguing about other stuff 30 years ago, and didn't have time to argue about this stuff as well. Or maybe because most of us were too embarrassed to mention we had totally changed our minds on this, and that the views we used to say were uneducated views were in fact right after all. So we silently changed our beliefs, and tried to forget we had ever believed anything different. But not all of us noticed this change in beliefs, and still thought everybody else believed what we all used to. So now we are re-fighting the old ground we should have fought properly 30 years ago.

There are 4 possible positions to take on the debt. One of them doesn't make sense; the other 3 do. Which of those 3 is right is an empirical question.

Here are the 4 positions. I gave each one a name. I made up the quotes.

1. Abba Lerner. 'The national debt is not a first-order burden on future generations. We owe it to ourselves. The sum of the IOU's must equal the sum of the UOMe's. You can't make real goods and services travel back in time, out of the mouths of our grandkids and into our mouths. The possible second-order exceptions are: if we owe it to foreigners; the disincentive effects of distortionary future taxes; the lower marginal product of future labour if the future capital stock is smaller.'

2. James Buchanan/uneducated person on the street. 'The national debt is a burden on future generations of taxpayers. Foreigners are basically irrelevant. Any second order effects of distortionary taxes and lower capital stock are over and above that first order effects of the taxes themselves.'

3. Robert Barro/Ricardian Equivalence. 'The national debt is not a burden on future taxpayers (except for the deadweight costs of distortionary taxation) but only because ordinary people take steps to fully offset the burden on future generations by increasing private saving to offset government dissaving and increasing bequests to their heirs to offset the debt burden.'

4. Samuelson 1958. 'If the rate of interest on government bonds is forever less than the growth rate of the economy, the government can run a sustainable Ponzi finance of deficits, where it rolls over the debt plus interest forever and never needs to increase taxes, so there is no burden on future generations.'

I personally was taught 1 as an undergraduate. And I believed in 1 until about 1980, when I spent some time reading Buchanan and Barro arguing with each other. And I worked 4 into my own beliefs soon after.

And now, I believe 1 is false. The truth is some sort of mixture of 2,3, and 4. What precise mixture of 2,3,4 is true is an empirical question. My prior is one third-one third-one third.

Until last week, I thought that almost every macroeconomist had now realised that 1 was false. And I wrote a post arguing 1 is false. My post was triggered by Paul Krugman's recent post, which I interpret as saying that 1 is true. Paul wrote two later posts too, which I interpret as also saying that 1 is true.

Many other bloggers have now waded into this debate. Good! Finally (I was scared we were going to keep on ignoring this question)! Bill Woolsey. Greg Mankiw. Don Boudreaux. Karl Smith. Daniel Kuehn. Jim Hamilton. Noah Smith. (I must have missed some. Sorry.)

And look, just because deficits have costs doesn't mean we shouldn't do them. Like a lot of things, deficits have benefits too, and sometimes the benefits are bigger than the costs. But we shouldn't ignore those costs, just because we think the benefits are bigger than the costs.

And what we need to do more work on is this: we know Samuelson is right, if we know for sure that the interest rate on bonds is less than the growth rate of GDP forever. That is a sufficient condition for Samuelson being right, but I rather doubt it's a necessary condition. What if the interest rate will probably be less than the growth rate part of the time for a rather long time? Could Samuelson still be right, or at least partly right?

I really wish I could get my head around that question, but I can't. My hunch is that NGDP bonds would play a role in the solution, because a bond indexed to nominal GDP would resolve the uncertainty of whether the interest rate would be above or below the growth rate of GDP. And if we got the duration of those NGDP bonds right, we ought to be able to get around the problem of r being sometimes greater and sometimes less than g. Somehow, it just must be possible to eat the Samuelsonian free lunch, even if it's only a temporary and uncertain free lunch. And NGDP bonds just must be the way to eat it, somehow.

And if I could only get my head around that question, I might come up with something that would make both Scott Sumner and the MMT guys very happy indeed. Which would be neat. Plus, as a side-benefit to making both those guys happy, it might be very good for the poor ignorant uneducated slob on the street too.

But my brain just can't figure it out, yet. Maybe some of you younger, keener, brighter, people could work on this?