When to talk about spending?
The government thinks that the economic situation is too uncertain to have a review of public spending priorities this side of the election.
It is getting rather difficult to find anyone who agrees.
You would expect the Conservatives to call for more detail on future cuts. The tougher things look under another Labour government, the less scary things look with them in power.
But today, the OECD also said that it would like to see more detail on how the government's "efficiency savings" in 2010-11 were going to be achieved. And, though I was on holiday last week, I'm pretty sure that Mervyn King repeated his call for a meatier plan to cut public borrowing as well.
Politically, of course the government would rather not provide any more meat for the opposition than it has to. And economically, the outlook is indeed highly uncertain. But if you take seriously the idea of a Comprehensive Spending Review, the experts I've spoken to aren't convinced that the uncertain times are a good reason to delay.
When Gordon Brown launched the first such review in 1997, it was supposed to be about stepping back from all the ups and downs of the political and economic cycles, to think strategically about the long-term priorities for public spending.
Would it be helpful to be doing that kind of strategic thinking right now? The Conservatives are not the only ones who think that it would.
Regardless of what happens to the economy in the next couple of years, we know that a major fiscal tightening is on the cards over the course of the next parliament. Taking the PBR and the Budget together, the "headline" tightening already announced by the chancellor comes to nearly 6.5% of GDP by 2017-18, or £90bn/year in today's money.
In today's report on the UK economy, the OECD says the government "could do considerably more to accelerate its programme of fiscal consolidation, dependent on how the economy evolves."
It also says that:
"[E]xperience in other countries suggests that a focus on expenditure cuts, rather than revenue raising, is associated with more successful consolidations, particularly when coupled with explicit expenditure rules, as it is more likely to result in lower interest rate spreads and instil confidence by signalling a strong commitment by the government to a robust fiscal consolidation. The delivery of the consolidation will require specifying the 'value for money' savings beyond 2011-12 in the upcoming Spending Review."
Or not so upcoming, as it turns out. But the OECD economists clearly think that spending control is where the budget-tightening action should be, coming out of this recession. That's because, in the past, tough spending control has tended to win most points from investors, and thus larger falls in long-term interest rates for every pound that is cut.
As the OECD readily admits, we don't know when we will come out of the recession, or how quickly. Is that a reason to delay thinking about cuts?
Lord Mandelson thinks so. He told the BBC this morning that "any spending review now would be based on entirely speculative projections of what economic growth will be". Well, yes. But they always are. That's what economic forecasts are. Speculative. And, as we are learning, often wrong.
It is quite true that if the economy takes longer to recover than the government now expects, the outlook for spending would probably have to change as well. For one thing, borrowing could rise even further, and the government might find itself having to tighten more, for longer.
Robert Chote, the Director of the IFS, told me: "if you're not sure whether you're going to have to take a medicine for six or nine months, that's not a very good reason to delay starting to take it altogether".
Everyone accepts that the fiscal tightening we're looking at is a job for more than one parliament. (For its part, the OECD says that returning to a ceiling of 40% of GDP for public net debt "is unlikely to be feasible until beyond 2020.")
You might have to delay or to rejig the plans in light of developments - heaven knows, that's what the government has always done before. As Chancellor, Gordon Brown was infamous for fiddling with the spending plans, spending review or no spending review.
But for the IFS and other experts, none of this is a good reason to delay thinking about how government is going to be reshaped to fit a much smaller amount of cloth.
I'm not sure anyone in Westminster seriously ever expected a spending review before the election. But as I said at the start, I've been looking for solid economic reasons to delay. I haven't been able to find very many. Perhaps you can.