The argument against the new surveillance schemes floated in response to the Islamist terror menace is essentially threefold, as follows.  First, that any new powers will be abused by the authorities.  Second, that they won’t work anyway.  And third, that even if they did they would be excessive – disproportionate to the threat we face.  How does this case stack up?

Over the weekend, Jonathan Evans, the former Director-General of MI5, said that he finds “perplexing” claims that new powers are “merely an excuse for sinister forces within the state to snoop on the harmless activities of ordinary people going about their private business”.  But whether or not they would be an excuse is beside the question – which is whether or not the powers would be misused, whatever the original intention of asking for them may have been.

For example, the Regulation of Investigatory Powers Act (RIPA) was undoubtedly abused by local authorities – unless, that is, you believe that councils should habitually spy on families suspected of flouting bin rules.  And it is currently being exploited by the police, who have gone through journalists’ phone records and bypassed judicial scrutiny in doing so.  The state’s instinct is to expand its activities.  The first argument against new powers has force.

On this site last week, Andrew Bower wrote that “the technical implications of the encryption ban are so wide-reaching the policy would be a joke in the technical community if it weren’t a serious proposal by a party of government”.  The nub of his article was that reading letters or tapping phones (which the security services are already empowered to do) is one thing, but accessing encrypted material is quite another: it would be impossible – or, at the very least, the cure would be worse than the disease.

It is certainly hard to see how government could in effect control the internet here amidst an age of free and open operating systems.  The implications of the plans are a ban on such systems, visitors to the country leaving their smartphones at airports, the curtailing of internet use and academic research, and the flight of business from Britain – plus the exposure of our e-mails and internet reading to criminals, as they master the special features that have deliberately been built in to software by the state.

So the second argument is very strong, too – though it doesn’t cover schemes that are easier to implement, such as obliging mobile phone companies and internet service providers to retain the records of internet, mobile phone and social media activity for a year.  This is one of the main proposals of the Communications Data Bill, a.k.a the Snooper’s Charter, which also floats a “request filter” – which would enable the security services to trawl the personal data held by these private companies.

This third claim – that such powers would be excessive – is persuasive, too.  The security services already cannot fully monitor the growing list of suspects returning from Syria or who they know have been radicalised to extremism online: how on earth, then, could they simultaneously be expected to probe millions of text messages and e-mails sent by citizens who aren’t even suspects – even if doing so didn’t endanger civil liberties in the first place?

Furthermore, most terrorists turn out to have been on the security service’s radar in any case.  The Kouachi brothers were – the perpetrators of the Charlie Hebdo murders. So was Michael Adebolajo, one of Lee Rigby’s killers.  So was Mohammed Siddique Khan, the ringleader of the 7/7 gang.  New powers aren’t needed, the argument runs – just more money and people for surveillance.  And in any event, the terror threat, though serious, is manageable.

This last point is the chink in the formidable armour of the case against new powers.  To date, successful terror attacks by Islamist fanatics have been rare and infrequent in Britain – thanks to the vigilance of the security services.  The balance of the argument would be different were matters to change.  Most of us are a very long way away from being affected by terror, but that this could change is not inconceivable.  Saying No to new powers is one thing; saying Never is quite another.