A jury gives no reasons for its verdict, its members cannot be questioned and — barring some legal technicality or dramatic new evidence — its verdicts cannot be challenged on appeal. It is positively encouraged to have regard to the "demeanour of witnesses" so if it wishes to decide cases by the defendant's smirk, or the irritating way the investigating police officer answers every question with an unctuous "My Lord" it is perfectly free to do so. It is a rationalist's nightmare. There are limits, of course: the jury that used an ouija board to decide the fate of a murderer was criticised by the Court of Appeal (not that it helped the defendant: the jury's, or perhaps the spirit world's, verdict was triumphantly reasserted at the subsequent retrial).

Nevertheless, despite their glaring imperfections, juries remain one of the few popular parts of a justice system that generally fails to command the enthusiasm of the public. It is popular not because it is rational but because it is human. Humans are often irrational, often prejudiced and often stupid. Judges too, being human, display all these traits. But bring 12 randomly selected people together into a jury and they tend to become deeply conscious of the solemnity of their task, and to become more sensible, level-headed and fair-minded. Their prejudices are not removed but they are at least the prejudices of society as a whole and the worst ones tend to cancel each other out.

On the other hand, elevate one person onto a dais and address them for year after year in language that hovers somewhere between the respectful and the fawning, give them enormous power over others and they tend to become less sensible, more prejudiced and less fair. Lawyers even have a word for this incurable affliction that affects most judges to some extent: "judgeitis". Put a perfectly decent lawyer on the bench and watch as he or she gradually becomes more pompous, more self-righteous, more prejudiced and more distant from those that he or she is judging.

Next year we will celebrate the 800th anniversary of Magna Carta and the need for vigilance in the defence of our liberties has never been greater. The jury system is a central part of that defence. Juries protect our freedom from unwarranted prosecutions, from government bullying and, occasionally, even from the judges themselves. No one put it better than Thomas Jefferson in 1789:

"It is left . . . to the juries if they think that the permanent judges are under any bias whatever in any cause, to take on themselves to judge the law as well as the fact. They never exercise such power but when they suspect partiality in the judges; and by the exercise of this power they have been the firmest bulwarks of English liberty."