Government measures to outlaw non-violent extremism are both unworkable and undesirable.

I never intended to be a lawyer. My degree was in English literature. I read a lot of poetry at an impressionable age. I remember being struck by a line from TS Eliot: ‘I gotta use words when I speak to you.’ I took it as extolling verbal precision, a value that provides a good crossover to the law.

A short course in textual criticism might be a good idea for both David Cameron (politics, philosophy and economics) and Theresa May (geography). They are in a pickle – if they survive the Brexit vote – over a well-trailed promise of legislation to counter dedicated ‘non-violent extremism’.

The prime minister has been expressing concern about non-violent extremism for some time. He told the UN general assembly a couple of years ago that: ‘As evidence emerges about the backgrounds of those convicted of terrorist offences, it is clear that many of them were initially influenced by preachers who claim not to encourage violence, but whose world view can be used as a justification for it.’ The answer is to stand firm against ‘the peddling of lies: that 9/11 was a Jewish plot or that the 7/7 London attacks were staged’.

The government defines terrorism as ‘vocal or active opposition to fundamental British values, including democracy, the rule of law, individual liberty and mutual respect and tolerance of different faiths and beliefs. We also include in our definition of extremism calls for the death of members of our armed forces, whether in this country or overseas’. Since 2011, this has been the basis of the government’s Prevent strategy. Any lawyer (let alone poet) might be happy for this to be a working definition for government policy – but not for translation into law. This will happen if it is incorporated into some ‘anti-extremism disruption order’, which might be civil in form but ultimately enforceable through the criminal courts.

Note that the current definition is in two parts. The second (calling for the death of soldiers) covers statements already likely to be seen as an incitement to murder under section 16 of the Offences against the Person Act 1861. The first is, frankly, a nightmare. To take a topical example, would refusal to bake a cake to celebrate the marriage of a same-sex couple combined with a strongly held view that homosexuality was sinful be extremism under the definition? It looks likely. You might not agree with such conduct but does it help to throw an ASBO at it?

Cameron has talked several times of denial of Al-Qaida responsibility for 9/11 as the sort of extremist assertion he wants to ban. I have read the US 9/11 Commission Report pretty well cover to cover: I commend it. It is thoroughly convincing as to its factual account. Once the missing pages on Saudi involvement are published, it leaves little room for doubt. Yet that has not stopped 100,000 New Yorkers signing a petition for a new investigation. Nor does it stop organisations such as Architects and Engineers for 9/11 Truth asserting that ‘the official story of what happened at Ground Zero simply cannot be true’.

So, the current definition of extremism will catch the terrorist with an underhand intent but also the sceptical non-believer who may be hard to understand but is not intending violence. And not only is any criminalisation of ‘non-violent extremism’ unworkable, it is undesirable. As terrorism in Northern Ireland was winding down, Lord Lloyd of Berwick was asked to report on what permanent anti-terrorism legislation should be enacted.

He came up with four principles: it ‘should approximate as closely as possible to the ordinary criminal law and procedure’; ‘additional statutory offences and powers’ might be ‘justified, but only if they are necessary to meet the anticipated threat’ and ‘must then strike the right balance between the needs of security and the rights and liberties of the individual’; ‘the need for additional safeguards should be considered alongside additional powers’; and ‘the law should comply with the UK’s obligations in international law’.

The problem with powers beyond these constraints is that they feed into the victimisation narrative common to most terrorists and their wider circle of supporters of unjustified discrimination by the state. Promises to silence dissent may play well in the Cotswold villages of Cameron’s constituency or the Thameside commuter towns of May’s. In Luton, Birmingham and places much more important in the potential breeding of terrorist violence, reaction will be much more hostile, even by those who are by no means terrorists.

So, use a wide definition for the purposes of a policy response but a fundamental value of the English legal system is certainty: many shameful times, it has even trumped justice.

Eliot also noted that ‘it is strange that words are so inadequate’. That is generally when the thinking behind them is inadequate as well.

Roger Smith is visiting professor at London South Bank University and former director of human rights group Justice