David Bain has had his day in court and didn't take it. He didn't take the witness stand to tell us what he knows. He is the only living person who must know something of what happened in that Dunedin house in the days or the night before his family was murdered.

Something happened; there's been plenty of conjecture from his supporters. But why didn't we hear from the man himself?

He testified at his first trial but not at this one. Without his testimony, the second jury had an impossible task. The conflicting stories put to them by the prosecution and defence were based on different methods of reasoning.

The police built their case on forensic science - bloodstains and DNA matches on David Bain's clothing and the absence of any such traces on his father. The defence made its case on motive - Robin Bain had one if his daughter had told the family what she had told witnesses at the trial, and perhaps the family had decided to banish their father rather further than the caravan in the backyard.

Perhaps they were going to the police in the morning. Who knows?

His defence team did not have to establish it. They needed only to cast doubt on the police case and a reasonable motive for Robin Bain would do it.

But why didn't they make it stronger? They had the only real witness in the room to the internal dynamics of that family. Why didn't they put him on the stand?

Motive is a persuasive factor in the minds of most people but it is not nearly as persuasive in criminal law. In fact it can be a hindrance to a proper investigation; the obvious explanation is often not the truth.

Police do not have to establish a motive to press a successful prosecution, physical evidence connecting the person to the crime is considered more reliable proof.

All this is explained to juries at a trial but jurors are not lawyers. My guess is that, like the rest of us, they spend more time discussing possible motives than blood stains and other forensic detail.

Juries are also told they must read nothing into the decision of an accused person not to give evidence. Judges are very strict about this. It is a sacred element of English law that no one charged with a crime can be compelled to speak at their trial, and judges know that defendants who keep silent are probably obeying their lawyers' instincts rather than their own.

But it is a dubious element of law, and one which does not conform to natural justice.

When you or I act in a judicial capacity - as parents or employers or some other position of authority - we naturally demand an explanation from somebody accused of wrongdoing.

When a staff member makes a serious accusation against another, the boss does not give the accused an ultimate right to silence. There are many rights an employee must be given these days, including perhaps access to a lawyer, but sooner or later a satisfactory explanation will be demanded. If none is forthcoming is is natural to assume the worst. Criminal law differs from natural justice in this respect I suppose because the penalties are so much greater, often a loss of liberty, and proof must reach a higher standard.

And there is always a risk that an innocent person might be nervous on the witness stand and led by clever or aggressive cross-examination to incriminate himself. But David Bain is 37. Since emerging from prison he has impressed on television as a calm, collected man, surprisingly good humoured for someone who has been wrongly imprisoned for 12 years.

He was at university when the murders occurred. His interests were in music and drama. He was not dim-witted or unsophisticated then and he certainly does not appear to be now.

If he could face a cross-examination then, he could handle one even better now.

After all this time, and with so many incongruities in the case, the verdict of the second trial is unlikely to put arguments to rest. But I'm tired of conflicting theories.

Nobody knows what happened to cause this family's destruction, nobody but its lone survivor and even now, at a trial his supporters have campaigned long and hard to secure, they did not see fit to have him tell us what happened in his own words.

It is many years since I read Joe Karam's account of the case but I remember being frustrated by how little of Bain was in it.

The book was Karam's account of what he thinks happened: David's story it was not.

When so much public interest has been aroused I think we have a right to hear that story.

I hope we may yet hear it.