I’m afraid I did not respond to requests to talk directly about the debate over Gerald Friedman’s numbers. I think I can only cope with that kind of thing one country at a time, and there were better people on the case. But I suspect a key to seeing your way through the wider debate is to know when to be optimistic about economic growth, and when not to be.
Martin Sandbu, channeling Narayana Kocherlakota, is quite right that we should not discount the possibility that economic growth could be unusually strong over the next decade or two. There is a significant chance that some of the slowdown that has appeared to have occurred to trend growth since the Great Recession might be reversible, partly because many hysteresis effects are also reversible. Inflation may not respond positively to strong growth as it has done in the past.
That possibility is high enough that it should have a big influence on monetary policy, for reasons I and others have outlined many times. The cost of needlessly throwing away potential resources is much higher than the cost of small overshoots of an inflation target. For that and other reasons the Fed’s decision to raise rates - and the MPC's decision not to cut them - was a mistake, as is continuing austerity.
Does that mean we should hard wire this optimistic view into budget projections? Essentially no, because budget projections should be based on your central guess of what is going to happen rather than any best case scenario. Almost every politician thinks they have the magic ingredient that will lead to strong growth. There is nothing wrong in that, but they should hope for the best and plan for the ordinary.
Doing this imposes a discipline on the electoral process that is essential to stop some politicians pulling the wool over voters eyes. If a politician or party wants to go with optimistic numbers, then the debate should be about how reasonable those numbers are, so voters can see what is going on. A world where these things are not debated is a world where everyone promises the moon and no one is any the wiser.
Where the discussion can get confused is if we put these two things together, and note also that fiscal stimulus involving additional investment would be good for the world right now. But that should and is argued for without pretending that it can all be paid for with the taxes that will come rolling in as it happens. There is a rock solid case for paying for extra investment - investment in its widest sense including human capital - by borrowing, because future generations benefit from that investment. When real interest rates are as low as they currently are, you have to be ignorant, duplicitous or slightly mad to say otherwise.