Theresa May’s “enough is enough” speech is an attempt to reshape dramatically Britain’s policy to thwart terror after murderous attacks. Mrs May gave her most explicit pitch today to policing thoughts rather than acts. This is a bad idea. It rests on a strategy to counter ideology rather than one that counters terrorism. It penalises people for holding unspoken beliefs and promotes a form of thoughtcrime. Such a move would end up with Britain losing the fight against terrorism in a legal minefield of dogma and piety. Mrs May wants us to believe that we face a threat from doctrines that do not espouse violence but somehow mutate into terror by contingency. The conclusion of her speech is that a non-violent person who harbours anti-British, extremist thoughts – to be defined presumably by a future parliament – could be blacklisted, maybe even criminalised. This is a leap away from current policy, although Mrs May has been heading in this direction for years. It should worry us all. What of animal rights, ecological defence or anti-arms-trade activists who do not subscribe to violent belief systems when criminal acts – sometimes amounting to terror – have been carried out in their name? Will they be banned too?

Non-violent extremism is not just difficult to define. Even if one could ban so-called adherents from public positions this would hinder, not help, the fight against terrorism. Facing the threat of terrorism means the government is obligated to exploit every possible avenue to prevent it. Mrs May’s policies exclude cooperation with the very people who might be best placed as discouragers of terrorism: those who hold similar “extremist” beliefs but who are non-violent and are opposed to the methods of violence. Mrs May’s plans criminalise the eyes and ears you need to spot terror.

Then there is the need for unity. Even before the police publicly commented on the identity of the three attackers, the prime minister had linked the London Bridge massacre, last month’s Manchester suicide bombing and the Westminster terrorist outrage in March to a “single evil ideology of Islamist extremism”. Politicians should take care in the language they use. Both al-Qaida and Islamic State have tried to claim the mantle of Islam for barbarism and death. We should not play into their hands. Mrs May should not have lumped together murderers and peaceful Muslims who are simply observant rather than violent, even if it might be politically expedient to use such words. Ukip trades in crude Islamophobia and perhaps Mrs May wants to ensure she is not outflanked when a shiver of fright runs through the electorate. It may allow her to paint, surreptitiously, her Labour opponent, Jeremy Corbyn, as tolerant of extremists. But such words risk pulling people apart when they need to come together.

Have we learned nothing from our own history? Listening to the prime minister speak from outside Downing Street, it is troubling to conclude that we have not. The wickedness that saw ordinary Londoners enjoying a night out mown down in the street and stabbed in bars must not mean we trade too much liberty for security. More than four decades ago in October 1974, a terrorist bombing saw people lose their lives and scores injured within a week of a general election. A few months later MPs voted for the Prevention of Terrorism Act, containing emergency “temporary” powers so draconian they needed to be renewed yearly. What were meant to be crisis measures became permanent. Current legislation reproduces much of the earlier problematic law, along with more intrusive powers. The aim of terror is to scare us into changing the nature of our democracy. Promoting sweeping new measures with alacrity in response to terrorist acts is no way to proceed. Politicians should steer public sentiment on matters of national security. But they should do so in an atmosphere becalmed by reasoned debate; not in the frenzy of the final days of a general election.