Arthur Miller's "The Crucible" is one of the great plays of the twentieth century. A reaction to the McCarthy anti-communist campaign of the 1950s, it powerfully represents America's deepest Cold War anxieties and the fear of an invisible enemy - the famous "Reds under the bed".

In "The Crucible" we are back in 1692 in Salem, Massachusetts. Here a closed, puritanical community becomes obsessed with "witches in our midst". It is a story of betrayals, denunciations, jealousies and accusations. Collective paranoia has taken hold and victims must be found: only violence will satisfy the cold, cruel anger of the mob. The play ends darkly - the community has become hysterical, blood must be shed to calm the psychic frenzy, and the innocent must hang. And hang they do.

These days we don't exactly believe in witches, but the term "witch-hunt" is still in common use in English. Usually it refers to public accusation in the media in response to some deep anxiety that seems to build up in society - maybe as a result of religious, political or security fears.

In the Chinese Cultural Revolution of the 1960s, the slightest accusation of counter-revolutionary activities could result in a teacher ending up on a pig farm, shovelling manure for many a long year. Teenage students joining the Red Guards suddenly had a way to get back at teachers they didn't like. Or maybe they simply found themselves in the possession of a new kind of power that met some sadistic impulse. They could be cruel, without having to be accountable.

So what is it that leads a group of people to destroy an individual publicly? From where does humankind get this impulse? Reasons might include a desire for justice or revenge, the need to maintain group harmony or even the opportunity to express natural innate violence. Psychological explanations include in-group persecutions of the outsider, or Freudian ideas of unconscious projection of negative emotions onto a convenient target.

Witch-hunts are all around us. As I write, here in the UK a scandal is being revealed in the media. Politicians have been claiming excessive expenses, and the newspapers have found out.  Every day a new target is selected, and their expenses, either shockingly high or embarrassingly trivial, are revealed. Careers are, perhaps rightly, terminated. But the interesting thing is not the scandal itself but the way public anger is expressed. TV audiences boo and hiss, people jump up in political meetings, faces twisted in rage, to denounce "thieving toads". Anger is everywhere.

It is difficult to assess how real this anger is. The emotions are surely genuine, but they are expressed in a GROUP context, and how much does the individual truly own the anger of the group?

Group anger is a very unpredictable thing, and that is why we need the rebels, the individualists and the free thinkers to tell the group when it has gone too far. Of course, this could be a very dangerous position to be in, since the individual might quickly become a target him- or herself.

Challenging the group to support the victim of a witch-hunt requires great reserves of physical and moral courage. However, going against the herd has a vital role in a civilised society. And it is a role that is just as important today as it always was.