After three years and two missed deadlines, we must leave the EU on October 31." Thus did Boris Johnson launch his campaign for leadership of the Conservative party at a pivotal moment in the UK's history. Choices made now will reverberate for generations.
Trusting Johnson's words is always foolhardy. But the implications are clear: if he cannot obtain a better deal than Theresa May's (which the House of Commons has rejected three times), he must either turn tail or choose a no-deal Brexit.
Leaving the EU is in itself damaging: it will worsen the UK's trading opportunities and influence on its continent and the world.
That this is happening when the western order is disintegrating and a new cold war between the US and China is emerging makes the UK even more vulnerable. But a no-deal option would be far more damaging than leaving the EU in an orderly manner.
The fact that senior politicians are contemplating it renders them altogether unfit for office.
What then is wrong with a no-deal Brexit? Here are six answers.
First, it will be disruptive. Nobody knows quite how disruptive, for the obvious reason that no advanced country has terminated its principal trading agreements overnight, in peacetime.
A large proportion of UK businesses, especially small and medium-sized ones, are unlikely to be ready, for the good reason that they still do not know what will happen.
A confidential cabinet note has recently warned that the country will not be ready on October 31.
Second, EU co-operation in a no-deal Brexit, while real, would be limited. The EU would ensure basic road, rail and air connectivity. Financial continuity should also be ensured. It would try to manage the fallout of a no-deal Brexit on the island of Ireland.
In all, it would not set out to "punish" the UK (contrary to what many Brexiters believe). The UK would just be treated as a "third country", across the board. But the jump from being a full EU member to this very different status is sure to be disorderly.
This is why standard trade agreements have long transition periods — far longer, in fact, than in the withdrawal agreement signed by May.
Third, the long-run costs of being without any deal with the EU would be substantial. Some important sectors — cars, for example — would face high tariffs (10 per cent in this case). The operation of complex supply chains would become substantially more difficult.
It is very unlikely that UK financial regulation would be deemed "equivalent" to that of the EU in such circumstances, with significant adverse implications for UK-based companies.
The ability of UK-based professionals to supply services in the EU flexibly would probably disappear.
Fourth, for such reasons, no deal could not possibly be the end of negotiations, but a shift to new ones from a far weaker position.
The EU would remain a far more important trading partner to the UK than any other. Reaching a good trade deal with the EU would remain a vital UK interest.
Yet, to make progress, the UK would still have to satisfy the EU on the three principal areas of the withdrawal agreement: money, treatment of EU citizens and the Irish border.
Fifth, the EU knows that no deal would weigh more heavily on the UK. Yes, the absolute costs might be similar to both sides.
But the gross domestic product of the rest of the EU is almost six times as big as the UK's and its population almost seven times as big. The cost per unit of GDP and per person would be far bigger for the UK.
The EU's view that the UK will come to terms is one reason why no UK leader will persuade it to change the withdrawal agreement.
Not allowing the UK "to have its cake and eat it" is also an existential interest of the EU, since the alternative might be the end of the project in which its leaders passionately believe.
Sixth, leaving the EU without a deal would impair the UK's credibility as a partner, for everybody. It would make it more difficult for it to co-operate with European neighbours on such issues as security or science. It would devastate the country's residual reputation for competence, stability and probity.
No deal is either lunatic or a confidence trick. It is not a policy any sane or decent politician could even consider.
Of course, in a country whose dominant politicians are now Nigel Farage, Boris Johnson and Jeremy Corbyn, this can no longer be ruled out.
But the fact that this is the case merely demonstrates the disappearance from British politics of every pattern of decent and sensible behaviour that once made it admired.
I would now rule nothing out, even the proroguing of parliament to eliminate its ability to block a no-deal exit. But if such an exit did happen, a greater deluge would surely follow.
Written by: Martin Wolf
© Financial Times