In 2015, Conservative MP and leading Brexiteer Owen Paterson declared: “Only a madman would actually leave the market.” He was talking about Britain's relationship with the European Single Market. And he wasn't the only prominent Brexiteer making such arguments. Remember what we were told before the 2016 referendum? Daniel Hannan: “Absolutely nobody is talking about threatening our place in the Single Market.” Nigel Farage: “Wouldn't it be terrible if we were like Norway and Switzerland? Really?” He repeatedly pointed to their prosperity and independence as examples Britain could follow. Matthew Elliott, chief executive of Vote Leave, described the Norwegian or EEA option as potentially attractive. Even the Conservatives' 2015 election manifesto declared: “We say: yes to the Single Market.” Single Market participation and Norway-style relationships were plainly part of the Brexit debate. And then Britain left not only the EU, but also the Single Market and Customs Union. The economic consequences have been substantial. The Office for Budget Responsibility estimates that Brexit will leave Britain's economy around 4% smaller in the long run than it would have been had we remained in the EU, with exports and imports approximately 15% lower than otherwise. Goldman Sachs estimates the economy is around 6% smaller than it would have been. Research published by the US National Bureau of Economic Research puts the reduction in GDP per person at approximately 6–8%. These are not claims that Britain's economy has literally shrunk by those amounts since 2016. They measure the economic growth and prosperity that researchers estimate we have lost compared with remaining. Cambridge Econometrics separately estimated that by 2023 Brexit had reduced annual UK economic output by approximately £140 billion, with 1.8 million fewer jobs than in its no-Brexit scenario. And behind those figures are businesses facing additional paperwork, exporters confronting regulatory barriers, reduced investment and opportunities that Britain could otherwise have enjoyed. This is why the argument about returning to the European Single Market has become so significant. The Liberal Democrats now advocate rejoining both the Single Market and Customs Union. Their argument is that restoring easier access to Britain's largest neighbouring trading market would reduce costs, encourage investment, increase trade and create economic opportunities. Economic research supports the expectation of gains, although estimates vary. The important qualification is that rejoining would not instantly undo a decade of lost investment or automatically recover every percentage point of GDP lost to Brexit. But this debate is about more than GDP. European integration also means opportunities for people: young Britons studying in European universities, workers building careers across borders, businesses sharing ideas, scientists collaborating, and families maintaining relationships without so many bureaucratic obstacles. Restoring freedom of movement would restore reciprocal opportunities for British citizens across Europe, not simply permit Europeans to come to Britain. There is also a wider question about the kind of relationship Britain wants with its neighbours: one based on greater economic and social cooperation, shared opportunities, scientific progress and mutual understanding, or one that maintains many of the barriers erected after Brexit. A decade after the referendum, the evidence gives Britain a much clearer picture of the economic costs of leaving. And Owen Paterson's words from 2015 remain a reminder of just how different the Brexit debate was before the referendum.