The proposed anti-terror laws raise questions about the authorities' reliability, says Paul Vallely

I remember being told as a schoolboy that, during the Second World War, those who professed themselves to be conscientious objectors were asked a simple question. "If a Nazi burst through that door with a gun, and was about to shoot your mother and sister dead, would you pick up this gun and shoot to stop him?"

I don't know whether such a question was really asked, but you can see the force of it. It confronts us with the reality of our political principles. My response would be that I would shoot, if I had the presence of mind, and wasn't paralysed by the horror of the situation.

So let's take honesty a step further. "If you were utterly convinced that a man in custody was plotting some terrorist outrage, but at that moment were unable to prove it, would you lock him up until you could get that proof?" Again, I have to admit that I would. But the devil - or perhaps in this case the caveat is more angelic - is in the detail.

"Until you could get that proof. . . " How long should we be prepared to extend that "until" - for a set number of days? If so, we are setting aside principle for pragmatism; and whether that period is 90, 60, 45, 28, 14, or even a single day becomes arbitrary in one sense. Or should the period be extended until the certainty of the detainer's inner conviction - and his ability to continue to convince a judge - begins to wane? And what is a judge to make of an application that says: "We have 60,000 phone transcripts to go through, and we're only halfway"? When, should the judge's patience begin to run out, and why?

The issue does not turn, I suspect, on principle, but on the level of trust we have in those who make such decisions. This is where the Government is on such shaky ground. It is interesting that some politicians who were initially sceptical conceded that the police, in their private in-depth briefings, have made a good case why changing times and changing crimes mean they need to hold suspects for longer before being able to charge them. But the key question is not "Is there a case?" so much as "Can we trust the authorities to use this measure properly?"

After a decade in office, it is perhaps an inevitable dynamic in our political process that trust in any prime minister will diminish. But there have been particular blows to trust on the issue of security. The whole business of going to war in Iraq has been the most significant, with its dodgy dossiers and non-existent weapons of mass destruction. The publication this week of the memoirs of the former British ambassador to Washington is only the latest reminder of all that, with its mixed portrait of Tony Blair as clever with the global politics, but too detached from the policy detail.

The questions about trust go well beyond Mr Blair. The shooting dead of an innocent Brazilian at Stockwell tube station by police hunting suicide bombers was far more than a tragedy for the dead man's family. It struck a blow at public confidence in police competence. But it may well be something apparently trivial that is the most damaging thing here. Remember Walter Wolfgang, the 82-year-old thrown out of the Labour conference for shouting "Rubbish!" at the Foreign Secretary? To stop him re-entering the hall, he was arrested - under the Prevention of Terrorism Act.

In the end, the key question may prove not to be "Are these powers necessary?" but "Can we rely on their not being abused?" The public's answer to that question, unfortunately perhaps, is likely to be a resounding no.

Paul Vallely is associate editor of The Independent.