If McGuinness told the truth he'd end up in jail
There is a simple reason why Martin McGuinness doesn't come clean about his past: he'd end up in jail and not the Aras or Stormont if he was entirely open and stood over his comments.
The fact is that no general amnesty has been granted for offences committed during the Troubles.
There has been nothing like the South African Truth and Reconciliation Commission and, as a result, we have senior politicians like Mr McGuinness who have to be very careful what they say.
When my wife and I wrote a biography of him, requests for interviews, or fact-checking sessions, were met with letters from his solicitor, Barra McGrory, now tipped to be our next Director of Public Prosecutions.
Mr McGrory told us that his client would not be co-operating.
Mr McGuinness was probably acting on legal advice when he refused to tell the BBC's Mark Carruthers when he left the IRA.
Instead, he referred the journalist back to his comments at Lord Saville's Bloody Sunday Tribunal and refused to repeat them on air.
Under the rules of the Saville tribunal, he can't be prosecuted for anything he said at its hearings.
He claimed to have left the IRA in the early-1970s. In other interviews, he said it was 1974.
That's because 1974 was the year he was imprisoned by Dublin's Central Criminal Court for 12 months for IRA membership.
Double jeopardy rules mean that he can't be tried again for the same offence.
If he confessed to a later date for leaving, he would, in theory, be at risk of prosecution.
Mr McGuinness has to give predictable answers to such questions, but it doesn't mean the questions are illegitimate in the first place.
Anyone standing for high political office, particularly head of state and of the armed forces, must expect to have his pedigree probed by opponents and the media.
And Mr McGuinness has to live with the fact that no one really believes he left the IRA after Bloody Sunday, a time when people were generally flocking to join it.
On past form, it doesn't have much effect on voters. Gerry Adams topped the poll in Louth in spite of a public campaign by the daughter of Jean McConville, accusing him of complicity in the secret burial of her mother.
People expect this sort of thing to come up for republicans of that generation.
Voters seem prepared to judge Mr McGuinness and Mr Adams on their recent success in ending the IRA's war rather than their earlier role in keeping it going.
However, Mr McGuinness can be undermined by what he says during the election campaign.
For instance, his description of journalists and politicians who questioned him as 'West Brits' has a fanatical and paranoid ring to it which would make a PR adviser wince.
As soundbites go, it was more damaging than anything anyone else has said about him so far.
Candidate McGuinness is stuck with his historical baggage, but he can control what he says right now.
'No one really believes that he left the IRA after Bloody Sunday, a time when people were flocking to join it'