On Sunday Radio 4 will “bring together some of those whose lives were changed for ever by the Brighton bomb in 1984”. One whose life was changed for ever, Lord Tebbit – whose wife was paralysed in the blast – has condemned the programme and refuses to meet the bomber Patrick Magee.

Few would dispute Lord Tebbit’s prerogative to withhold forgiveness. But many would regard his stand as overlooking the costs required to advance peace. Symbols matter. Reconciliation among former adversaries is painful, but why demur if it helps to sublimate violence? The abandonment yesterday by the Ulster Volunteer Force of the armed struggle surely marks success in drawing into political activity those long-regarded as terrorists.

Unfortunately that conventional wisdom is exactly wrong. We should not only understand Lord Tebbit’s sentiments but applaud them. They are pragmatic politics of a type imprudently abandoned by governments of both parties.

It is welcome when groups formerly wedded to terrorism unconditionally embrace constitutional politics. The old Official IRA took that course when it metamorphosed into the nugatory Workers’ Party. In principle, a terrorist group with political and regional aims – say, Eta, Farc and perhaps even Hamas – rather than messianic ambitions, might follow that model. The question is how best to encourage it. The problem is that we cannot tell in advance which groups are malleable and which will merely exploit political concessions to extend their terrorist campaign.

The distinction is crucial, for eagerness to secure peace may lead politicians to attribute benign ends to bad people, and limited aims to apocalyptic nihilists. The late Mo Mowlam even urged negotiation with al-Qaeda lest we “condemn large parts of the world to war for ever”.

The most reliable way to distinguish between different types of terrorist group is to force on them directly the choice between violence and politics. It is a myth that IRA violence was militarily undefeated. Successive Irish governments repressed IRA campaigns with draconian efficiency. Recalling one such campaign from 1957, Conor Cruise O’Brien observed: “I am convinced that if the later IRA offensive, begun in 1971, had been met with the same determination . . . it too could have been brought to an early end.”

The Northern Ireland peace process itself has costs – notably the implicit message that obduracy and threats yield benefits. This has ossified a political division that bypasses moderation while rewarding Ian Paisley and Sinn Fein. In refusing to greet his attacker, Lord Tebbit is the voice of personal outrage – but also a reasoned politics for countering the forces of sectarian bigotry and violence.

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