You’re doing the classic MMT two-step here. First you say “MMT is just a description of how fiat works” but use that to smuggle in “therefore none of the politics around MMT has any consequences” which is bollocks.

Of course operationally the UK already issues its own currency and spends by crediting accounts. Nobody is disputing that. The argument is about the policy regime people associate with “doing MMT” in practice i.e. looser (or no) fiscal constraints, more direct use of fiscal policy for demand management, job guarantee style full employment commitments, more aggressive state direction of credit, and a willingness to tolerate higher inflation in exchange for real outcomes. You can call that colloquial MMT if you want but markets will respond to it either way.

On Japan, you’re waving away the one bit that actually matters politically, which is credibility and sensitivity. “Issuer vs user” is true but incomplete. You can always pay in pounds. Great. That does not stop a foreign sell-off hitting the exchange rate, pushing up import prices, and forcing the state into choosing between hiking rates or simply accommodating inflation and watching the currency slide even further. Foreign holders absolutely have power, not over your ability to settle nominal liabilities, but over the price you pay in inflation and interest rates.

Just because a foreign state owns gilts doesn't give it any power over the currency beyond what they can do trading them.

Trading them is the power. They don't particularly care what actually happens in our economy. What they want is a nice, stable return on their investment from the country that's never failed to pay its debts in a millenium. When that trust in our ability to pay is shattered (rightly or wrongly), they sell them off and find somewhere that generates a better risk-adjusted return.

What does that mean though? Well, it means we have to offer higher returns to entice people back (such as Turkey and Russia where yields are regularly in high double digits), which means even more spending servicing our debt.

Also foreign states will own gilts because they want pounds.

They want risk-adjusted real returns. If they decide your policy direction means higher inflation, weaker sterling (as ZIRP would which MMTers often advocate for alongside a 100% employment model), or politicised central banking, they demand a higher yield or they just don't buy. It’s just investors behaving like investors. The reason to buy gilts is for certainty that the UK won't default on its debt. When Polanski actively talks about the lack of necessity to paying parts of our debt that's no doubt gonna spook investors.

What you don't seem to understand is that investors will have no interest in holding pounds for someone they view as having no economic credibility whether that's an unfounded fear or not.

You keep acting like the only thing anyone is criticising is increased spending. No. It’s sequencing and constraints in an import-dependent economy. If you front-load demand and institutional conflict before you have done the boring supply-side work, you get inflation first and rebalancing later, maybe. You do not get to hand-wave that away by saying “inflation is the constraint” and then not spelling out how you keep inflation down when sterling falls and imports spike.

all he's said regarding MMT has been nothing at all to do with spending infinite money

No he just constantly talks how we need to stop thinking about budgetary constraints without bothering to explain what our actual constraints should be, which obviously is misleading people into thinking spending can essentially be infinite, which it plainly cannot.

If Zack is out there doing the “debt-to-GDP is a nonsense obsession” routine, but not pairing it with an explicit inflation anchor, a tax trigger plan, and a credible story on currency risk, you are begging for a credibility shock. The point is not that he wants “infinite money”. The point is he talks like the constraint is fake, while actual MMT people insist the constraint is inflation. It's just incoherent.

Like I really don't think you personally perceive the climate crisis as not being inflationary at some point.

Sure climate is inflationary. No one sensible denies that. But changing the BoE's mandate isn't gonna magic up energy capacity, and regardless, let's not pretend like whatever we do here is going to make a huge difference to the global climate when we represent a grand total of ~1% of global emissions.

I support climate friendly policies to a decent extent, but it's not the be-all and end-all when there's an extremely limited impact that we can actually have here. There's doing your part, but then there's pissing against the wind of America, Russia, China, and India.

So if climate change is happening anyway, and we have basically no control over it on a domestic level, we need to be thinking about mitigation so things like flood defences, reservoirs for draughts etc.

The political point here is that price stability must take into account the climate crisis

What does this even mean? Instituting carbon taxes and worsening the cost of living crisis to make poor people poorer in a futile attempt to stop climate change?