How long will markets tolerate our profound ignorance of May's economic policies?

I have not known a time when known unknowns about the economy and economic policy have been so legion, and yet markets - after their initial post-referendum jolt - have been so stable.

This is troubling, rather than reassuring.

What am I on about?

Well obviously it is good news that the extreme economic doomsayers among the Remainers have been proved wrong - thanks in large part to the extraordinary propensity of British shoppers to keep calm and carry on shopping.

We are an economy dominated by consumption. So households refusal to be spooked by the Brexit vote into hoarding cash has led "only" to a halving of the UK's growth rate (for now).

And the collapse in the pound has given a welcome boost to our too-small export sector.

But big investment projects are being slowed, cut back or shelved. Construction and housebuilding are heading for a (possibly mild) recession.

And it is highly unlikely that the export increment will in any serious way remedy our dangerous dependence on borrowing unsustainable amounts from the rest of the world to fund our lifestyles (our current account deficit, at circa 7%, is the widest since records began).

The point is we simply don't know how long the negative economic impact of Brexit will be sustained, or how severe it will be.

Because that will depend on the two most important known unknowns in government policy.

First, and as President Obama and Japan's premier Abe bluntly reminded Theresa May at the G20 over the weekend, the future contribution to jobs and investment of the multinationals that dominate our economy depends on their being given certainty that they will continue to have relatively frictionless access to the European single market.

And right now we have absolutely no clue how and whether that can be achieved.

Instead we have a fatuous dispute between the Brexit minister David Davis - who correctly said on Monday that the chances of the UK having continued membership of the single market are slim to none, if Mrs May sticks to her plan to repatriate control of immigration - and Downing Street, which eccentrically argues that all options are open.

As a business leader said to me this morning, "Help!"

Second, the fiscal stance of the May administration is all over the shop.

On the one hand, almost the first announcement by her and her new chancellor Philip Hammond was that they were abandoning George Osborne's commitment to generate a surplus by the end of the parliament.

But that is less a policy decision, and more a recognition that Brexit would reduce growth and therefore revenues from taxes.

It was to say that rather than impose yet more austerity and cuts when growth forecasts were not met, the Chancellor would allow the so-called automatic stabilisers (of higher benefit payments, and a reduced tax take) to operate.

However that tells us precisely nothing about whether Mrs May will adhere in the longer term to Osborne's plan to generate surpluses in all so-called normal years.

She rather hinted in her briefing to journalists in her official plane on Saturday afternoon that she was heading back to Osbornomics, when she said that it was important the government should "live within its means".

But in fact that is a precept consistent with all manner of different approaches to taxing and spending (Gordon Brown would argue he lived by it, and his so called golden rule permitted far more borrowing than Osborne's rules).

Or to put it another way, for the Chancellor and Treasury, deciding the appropriate taxing and spending measures for the autumn statement is subject to two massive uncertainties: we have no fiscal rule right now; and anyone who thinks they can reliably forecast the pre and post Brexit trajectory of the economy, and therefore tax revenues, is either a charlatan or a fool.

The autumn statement will at best be the equivalent of replastering a wall at night during a power cut.

Now, to return to where I started, it is a bit puzzling perhaps that - against that backdrop of inescapable ignorance about our economic future - that markets and investors are so calm.

Except that it isn't.

The explanation for investors' sangfroid is they are relying on the Bank of England to cut the cost of borrowing and print money at every possible bump in the road.

Which in practice, as I said right at the start (if you can remember that far back) should be troubling rather than reassuring.

Because for an economy - like the UK's which is excessively indebted and has been powered by borrowing for too long - to become even more hooked on the drug of cheap money, rather than reconstructing itself to generate sustainable rises in living standards through improving the competitiveness of industry, is the road to penury.