Although they are both “law”, criminal law and civil law are as different as football and rugby. There is not a single set of rules that applies across both types of law.

That means for instance that someone accused of violence who wants to plead self-defence will need to argue his defence differently according to whether he is in a criminal or civil court.

A recent case, Ashley v Chief Constable of Sussex Police, illustrates the difference between self-defence in civil law and criminal law. It is also an instructive demonstration of the broader differences between civil and criminal law.

As part of a crackdown on drug-related crime, the flat of James Ashley in Hastings, East Sussex, was raided by police at 4.20am on January 15, 1998. Although naked and unarmed he was shot dead by PC Christopher Sherwood. The officer said that he was acting in self-defence. He made a mistake in thinking Mr Ashley was armed, but, he argued, it was an honest mistake.

Christopher Sherwood was subsequently tried for murder. He was acquitted at the Old Bailey. It is a defence in criminal law to act on a mistaken fact. That mistake does not have to be a reasonable mistake, just an honestly held one.

The Chief Constable admitted that there had been negligence and accepted liability and the need for compensation. But James Ashley’s family pushed ahead with a civil action for assault and battery. This was what is sometimes known as a “vindicatory action” meaning that it is brought not to get compensation but to show that an important right has been infringed. Here, the civil action was taken to get a court judgment that James Ashley was killed as the result of an unlawful assault not mere negligence.

The Court of Appeal had to decide whether there could be a civil trial on the assault since PC Sherwood’s self-defence argument had already saved him at the criminal trial. The Court of Appeal said the civil trial for assault could go ahead. The reason is that in a civil trial the self-defence test for someone in PC Sherwood’s position is harder to prove than in a criminal case, so just because his defence succeed at the Old Bailey would not mean that it would save him in a civil court.

The difference is this. In a criminal trial someone pleading “mistaken self-defence” (I shot him because I thought, mistakenly as it turned out, that I was going to be shot) has only to plead an honest mistake. It is then for the prosecution to prove that he did not really have that belief. By contrast, though, in a civil case someone sued for assault can plead “mistaken self-defence” only if he can show that his mistake was both honest and reasonable. If he cannot show his mistake was reasonable he will be liable for assault. So, in a civil trial, the defence would have to show PC Sherwood’s mistake when he shot a naked unarmed man was not just an honest mistake but a reasonable one.

The reason for the different requirements of self-defence in the different types of court goes to the heart of the difference between civil and criminal law.

The ends to be served by the two systems are very different. One of the main functions of the criminal law is to punish criminal behaviour. But the law acknowledges that, in general, no one should be punished for the consequences of an honest mistake.

However, the function of the civil law of tort is different. Civil law is made to protect the rights of one person against others. Sometimes those conflict – like the rights of free speech and the right not to be libelled – so the law must strike a balance. In making that balance, the law uses reasonableness as a key standard. So, when it comes to the conflict between one person’s right to use violent self-defence and another person’s right not to be shot if he is mistakenly believed to be an attacker, the law uses the standard of reasonableness to judge the mistaken belief.

It is one thing to say that PC Sherwood did not commit a crime because he made an honest mistake. It would be quite another thing, and wrong, to say that simply because he did not commit a crime he did not commit a wrong in civil law.

In some ways it might look odd that the law on this point was not settled centuries ago. But the law is always developing. The common law is a living body of organic rules. The Ashley decision is a good example of how appeal judges can develop and clarify the law in a sensible and rational way in the context of real human drama. They do this not by making up new rules from thin air but by studying all the existing law and relevant cases, digesting the careful arguments of counsel, and coming to a thoughtful solution in line with the dictates of both law and life.

That is the great virtue of the common law. The rules are never left to ossify. The common law is a garden not a quarry. So laws are tended, watered with fresh thought, pruned and grown when necessary.

Professor Slapper is Director of the Centre for Law at The Open University

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