Some years ago I asked Alan Greenspan, who was just retiring as chairman of the Federal Reserve, whether he had ever worked out how to predict currency movements.
If there was one person in the world capable of doing so, you’d have assumed it might be Mr Greenspan, the man whose every word used to move the dollar.
And lo and behold, he told me he put a “huge amount of resources” into predicting those foreign exchange movements.
“We worked very hard, collected all the data – and we had far more than anybody else, because there was a lot of unpublished data on intervention and all that stuff which the Fed had.
“The rate of return on that investment of time and effort and people was zero.”
If the maestro of central banks had no way of divining where a currency was heading, what hope is there for the rest of us?
It’s a reminder that for all that we look for simple explanations for why a currency rises and falls, sometimes the explanation is far more complicated and chaotic than we might think.
Take Brexit and the pound.
Now, it’s highly probable that the pound’s movements over the past few years have had a lot to do with Brexit.
Broadly speaking, when the newsflow suggests the UK is heading for a softish Brexit, the pound tends to strengthen; when a no-deal Brexit looks more probable, it weakens.
And that makes a lot of economic sense.
A no-deal Brexit would almost certainly involve the erection of barriers to Britain’s trade with its biggest trade partners.
That in turn will probably diminish UK economic output which implies the UK is a less profitable bet for international investors.
It implies the Bank of England might have to cut interest rates, which again implies lower returns on UK investments.
Both of these factors should push the pound down.
But Brexit is hardly the only thing happening in the world today.
We are in the midst of a major monetary shift, as central banks, which up until recently were mostly raising interest rates (or talking about it) are now considering cutting them.
There are genuine concerns about a trade war, about a possible recession in parts of Europe.
These are events that are also likely to have had an influence on currencies.
But tempting as it is to ignore currency movements altogether, there are few particularly reliable and up-to-date measures on how well or badly Brexit is going down among financial investors, so the pound will have to do.
And the fact that the pound has now dropped below $ 1.23 against the dollar for the first time since Article 50 was triggered in March 2017 is a moment worth dwelling on.
In a sense, the odd thing is how long it’s taken for this to happen.
When Boris Johnson was confirmed as Tory leader, and subsequently formalised as prime minister, sterling actually rose somewhat (albeit from a low base).
As we reported at the time, this looked, on the face of it, a little odd.
Here you have a new leader who has pledged, if necessary, to take Britain towards a no-deal Brexit – words which have always pushed the pound lower.
Yet for reasons no one quite understands (see above) sterling remained pretty robust for a couple of days.
But eventually gravity reasserted itself and the pound fell further.
Indeed, my suspicion (again beware Mr Greenspan’s warning above) is that it will head even lower in the coming months.
Why? Because whether or not Britain ends up with a no-deal Brexit, Mr Johnson’s negotiating tactic between now and the end of October is to make absolutely clear that if need be he will take Britain through one.
The language will become even more heightened.
The drama will increase.
We will head towards the brink of a no-deal Brexit – because that is what Mr Johnson believes is the best strategy to ensure Europe actually considers reopening negotiations.
That sounds like a recipe for sterling weakness.
The only question is how low that pushes the currency.
After all, if other central banks are cutting rates (which, all else equal, pushes down their value too), then it’s anyone’s guess precisely where sterling would end up in relation to them.
Some strategists believe it could hit parity with the euro.
Some even think it could end up 1:1 vs the US dollar.
When you hear such forecasts, just remember that no-one – not the politicians, not the traders, not the negotiators or the central bankers – really knows.
In the meantime, the rest of us will have to learn to live our lives under the shadow of a less stable currency.
You don’t have to be a business owner or a tourist to find that unsettling.
A weaker pound pushes up the price of goods in the shops (since so many of them are imported).
It might make exporting more profitable, but since Britain comfortably imports more than it exports, that is not altogether compensatory.
For the first time in many years, real wages are rising at a comfortable rate of over 1 per cent.
But any increase in inflation puts that at risk, because if the cost of living rises faster than wages, we are all, by definition, getting poorer.
This is a big deal.
Indeed, the big story of the past decade is that we have undergone the biggest squeeze on real incomes since the Napoleonic era.
Much of the fury and frustration felt by so many around this country derives from that fact: we are all effectively poorer than we would have been.
At first that was down to the financial crisis.
But there have been false horizons since: just as real wages were recovering around 2016, the referendum happened and sterling fall so sharply inflation rose and the squeeze returned.
Now there is the risk of another false horizon: the more the pound falls, the more likely we will all see our standard of living shrinking all over again.
So this matters for everyone.