Does the financial crisis reveal that economists are at the leeches and mercury stage of their subject, and as a result policy makers and the public have every right to ignore what they say? Does the fact that economists working in finance failed to recognise the prospect of a systemic crisis, and that macroeconomists both took finance for granted and as a result failed to investigate financial-real links, mean that we should ignore what economists say when it comes to Brexit?
Speaking for my own subject, I think the financial crisis does raise serious questions about the methodology macroeconomists rely on, as I have explained at length elsewhere. But does it mean that everything macroeconomists have learnt in the last 80 years is virtually worthless, or at least no better than the opinion of the average politician? Why don’t we look at what has happened since the financial crisis.
Macroeconomists, having learnt the lessons of the 1930s, immediately recommended that policy makers do three things after the crisis: cut interest rates sharply, embark on fiscal stimulus and bailout banks. Policy makers took that advice in 2009, and as a result we avoided another Great Depression. Many said that rising government debt was sure to send interest rates on that debt rising: academic economists using basic ideas from Keynes said they would not and they were proved right. Many others said that Quantitative Easing (central banks creating money to buy government debt) would cause hyperinflation, but again academic economists looking at more modern New Keynesian models said that was nonsense and again they were right.
You might claim that in all this economists were just advocating what was obvious. The acid test came in and after 2010, when fiscal stimulus turned to austerity. What evidence we have suggests this move was opposed by a majority of academic economists, a majority that grew over time. There was a minority that supported austerity, at least for a time, and they gained a lot of publicity because politicians latched on to what they had to say. But the majority followed both textbook and state of art economics, and this majority was right. The recovery would have been stronger and faster if politicians had gone with this majority.
If we look back before the financial crisis at UK macro policy, we can again look at the record of economics compared to politicians. The obvious place to start is with the 364 economists, who despite all attempts by politicians and think tanks to suggest otherwise were right: tight fiscal policy in the 1981 budget delayed a proper recovery by over a year. We can look at the following recession in the early 1990s. A key driver behind that was the UK joining the ERM at far too strong an exchange rate. Here it gets personal. With colleagues at the National Institute I undertook what was acknowledged at the time to be the most comprehensive analysis of the appropriate entry exchange rate, and we argued that our entering at the then current rate was folly. We were ignored, and as a result the UK was the first to be kicked out of the ERM in 1992.
The next time the UK had to decide to join in this case the ultimate fixed exchange rate regime, the Euro in 2003, it was the economics that persuaded the Labour government not to join. In this case macroeconomic analysis played a critical role in making the right decision.
All this suggests to me that macroeconomics, if we compare it with medicine, is well beyond the bloodletting stage. It would be very surprising if we were not, given 80+ years of study and the huge amounts of data now available. Of course that does not mean academic macroeconomics will not make mistakes, and of course unconditional forecasters of the kind you read about endlessly in the papers will always get things wrong: our own models tell us they will. But when it comes to macroeconomic policy, experience suggests you are much more likely to get economic policy right if you ask an academic macroeconomist than if you ask anybody else. 
The other key thing to say is that the discussion above has virtually nothing to do with the long term impact of Brexit, which depends on international trade. The key bit of analysis that means trade with the EU cannot be simply replaced with trade elsewhere are gravity equations. Gravity equations do not come from theory but from the data: countries are much more likely, even today, to trade with near neighbours than far away countries after allowing for other factors. So when Rees-Mogg suggests that the Treasury must have fiddled the numbers, when the government’s analysis confirms those of other studies that Brexit will be costly for all of us, we know he is slandering civil servants for his own political gain. That he is also the favorite to replace May as leader of the Conservative party tells you all you need to know about the current mess the UK is in and why it is in this mess.
Of course we do not condemn engineering science when a new bridge wobbles or an oil rig fails, and we do not say that all medical science is nonsense when medics get things wrong, as they frequently do. But with economics, there are too many people who either want to replace the mainstream with their own school, or who like Rees-Mogg want to discredit economics because they suggest his preferred policy is harmful. As a result, whenever economics does make mistakes, as it will, there will be plenty of people around who want to bury the whole discipline. But when you look at all the evidence and not just one observation, as economists are trained to do, you find that you are better off following the advice of academic economists when it comes to economic policy than anyone else.
 The argument that academic economists should be modest or humble when giving their views should be seen in this light. They should certainly be honest about their own views compared to their colleagues, and they should also if they are given the opportunity express the uncertainties. But being modest and humble should never mean leaving politicians unchallenged when they proclaim economic nonsense.