Defence is the best form of attack

The problem is often posed this way: you've got a friend in your car who will die if you do not get to the hospital straight away. If you drive through a red light to get him there and are charged and taken to court, should you be able to claim that your crime was necessary in order to prevent a greater harm? In other words, should you be able to fall back on the legal defence usually known as necessity?

I love the necessity defence. It can be used in all sorts of cases, apart from murder, but it's always being trotted out by activists. We had to take that plane apart to stop it dropping bombs on Iraqi civilians. We had to climb that chimney to fight climate change. We had to break into that factory because otherwise they'd use those bombs in Palestine.

It's been used in two different cases in the UK: the Smash EDO bunch, who were accused of breaking into the EDO MBM arms factory in Brighton and causing £187,000 worth of damage in order to "decommission" weapons; and the Climate Nine, who entered Aberdeen Airport and occupied part of a runway for a few hours.

I love it because it demonstrates perfectly that our belief in the "rule of law" is a chimera. It encapsulates the impossibility of drawing neat lines around behaviour and saying, "If you step on this side you're right - if you step on that side you're wrong."

And it is quite correct; the law is a fraud. Take killing, and the fundamental proposition that killing is wrong. According to the law, yes it is, but it's more wrong if you thought about it beforehand (murder as opposed to manslaughter), less wrong if you were being attacked (self-defence) and it's not wrong at all if you're in an army (war), as long as the war is being conducted within the parameters of international law . . . And, oh, here we are again.

Unsurprisingly, lawyers and judges get very funny about necessity. Some argue that it should never be allowed because, as Lord Denning said in 1971: “If hunger were once allowed to be an excuse for stealing, it would open a door through which all kinds of lawlessness and disorder would pass."

But, equally unsurprisingly, juries are sometimes willing to go along with it. In 2007, a jury let the Fairford Two off after they had broken into an RAF airbase to ground B-52 planes and prevent, they hoped, potential war crimes against Iraqi civilians. In 2008, six Greenpeace activists were cleared of causing criminal damage at Kingsnorth Power Station after claiming they were acting to prevent the damage that would be caused by climate change. The Climate Nine, in the end, were found guilty. But the protesters in the Smash EDO case - who, as this goes to press, are waiting to hear the verdict in their case - claim the arms manufacturer is breaching legal guidelines and making parts for use by the Israeli military. They could not, they argue, sit back and let EDO MBM carry on.

Who knows what the jury will think when it makes its decision? Juries are renowned, after all, for what lawyers must see as infuriating unpredictability. You can often guess what a judge is going to do because he will try to stay within the parameters of a legal mentality. He believes that laws can cover the fathomless mysteries of human behaviour. The joy of the necessity defence, however, and the joy of a jury, is that we know better.