An effective apology has to be heartfelt. It must also satisfy the wronged person that the person apologising has done everything possible to redress the wrong and reveal the truth.
That is why David Cameron's apology for Bloody Sunday was so well-received in Londonderry and his equally frank apology for Pat Finucane's shooting did not satisfy Mr Finucane's family.
It is also why Gerry Adams's apology for the IRA murders of gardai put his credibility and sincerity under scrutiny.
Mr Adams (right) denies ever being in the IRA. If that is true, why apologise?
Why did he campaign for the early release of Garda killers?
Why not also apologise for the murder of RUC officers by the IRA while he is at it?
Why not disown republican ballads which commemorate past killings?
The questions only multiplied after he raised the issue in the Dail.
It smacks of political expediency; he couldn't condemn the murder of one garda without mentioning others.
Since their killing was against the IRA's stated policy, he could just about manage an apology - but not a condemnation.
Killing RUC members was IRA policy, so only a "recognition of suffering" is considered appropriate for those deaths.
"There is no hierarchy of victims," Sinn Fein insists. It wants equal treatment for the IRA's dead.
Yet maintaining these carefully modulated distinctions shows they operated one.