"We are leaving the European Union," said Theresa May in her letter to Donald Tusk, formally activating the withdrawal process. "We are not leaving Europe."

Her tone throughout the process has been courteous, statesmanlike and pro-European. She will implement the referendum result, but she also wants Britain to retain the closest economic, military and intelligence links to its EU allies.

Her moderation stands in marked contrast to the hysterical tone of her opponents. Scotland's first minister, Nicola Sturgeon, says that the Conservative leader is bent on "extreme Brexit." The former deputy prime minister, Nick Clegg, says she leads a "Brexit-at-all-costs Government."

It's worth reminding ourselves that Mrs. May, three-quarters of her Cabinet and two-thirds of her MPs campaigned to stay in the EU. Far from being dogmatic Euroskeptics, they are sincere democrats who accept that last year the British people instructed their leaders to recover national sovereignty. They aim to carry out that instruction in a way that maximizes the benefits to Britain and the rest of the EU. That may well mean replicating through bilateral treaties some of the arrangements that the U.K. must currently pursue as a member.

The difference is that Britain will now be an independent country, living under its own laws. Its relationship with the EU will be based on alliance, not absorption.

Pro-EU newspapers describe that change as a "step into the unknown," as though national self-government were a radical new concept. It's true, of course, that we can't know every detail of how our post-EU future will work out, though we can reasonably surmise that it will be more global, more deregulated and more competitive. Still, if you think about it, human history has seen a series of steps into the unknown. The Puritans who boarded the Mayflower at Plymouth were making a step into the unknown. The signatories to the Declaration of Independence were making a step into the unknown. They had no doubt that that there would be challenges ahead, but they wanted to face those challenges themselves. That is what independence means.

What we're seeing here is inertia bias: the human dislike of change. Opponents of Brexit are determined to see a future full of obstacles rather than opportunities. I see cheaper food; they see poorer farmers. I see freer trade; they see untrammeled corporations. I see a closer alliance with the United States; they see domination by Donald Trump. I see lower energy bills; they see pollution. I see independence; they see bigotry. I see democracy; they see mob rule.

For what it's worth, their predictions have so far been woefully wrong, but when does that ever prompt self-analysis? Rather than being glad that the promised post-referendum slowdown didn't happen — jobs, exports, shares, manufacturing and exports are all up — they have doubled down, promising that the apocalypse will be even worse when it arrives.

In fact, Britain's trade talks with the EU ought to be straightforward. We are starting from a position of zero tariffs and regulatory equivalence, so, in this instance, the inertia bias favors free trade. I am not aware of any serious politician on either side of the Channel calling for tariffs. Though there may be some regulatory divergence over time, this will happen only when the U.K. decides that the cost of applying EU standards to this or that sector outweighs the benefits of easier exports.

In her letter, May gently touched on the fact that security collaboration, like trade, is in the interest of both sides. Britain makes a disproportionate contribution to the defense of Europe, both in military terms — we have just deployed troops to Poland and Estonia, for example — and by sharing intelligence. We are one among 28 EU states, but we pay 20 percent of the military costs. One or two EU officials have complained that she brought the subject up at all, claiming that she is trying to "blackmail" the EU by tying security cooperation to a trade deal.

Actually, no European country has an automatic entitlement to be defended by British or, indeed, American taxpayers. Both English-speaking democracies have made that investment for 70 years because we value the security and prosperity of Europe. Mrs. May has every right to remind her EU colleagues of our commitment.

The U.K. has always wanted trade, friendship and alliance with its neighbors rather than political integration. Before the referendum, David Cameron tried to secure such a deal as an existing member. The EU refused, so we are now pursuing a looser arrangement from the outside. It really is that simple.

Dan Hannan is a British Conservative MEP.