In the 19th Century, journalist and barrister Walter Bagehot attempted to describe the concept of public opinion by giving a description of a man on a bus; an idea that was later taken up by the courts in order to better characterise the opinion and actions of a 'reasonable person'. This person, known as 'The Man on the Clapham Omnibus' can be a useful way to approach legal and ethical dilemmas.

What might the man (or woman) on the Clapham omnibus make of the case of Anders Behring Breivik, currently on trial for the killing of 77 people in Oslo and Utøya in July 2011?

After a week of detailed and harrowing testimony from Breivik, there might be many a reasonable view expressed on public transport. Some might argue that Breivik must have been sane to have organized and carried out a crime of that scale with such chilling accuracy and without getting found out. Others might question how any sane person could to do such a thing in the first place, especially hearing his reasons for doing so.

These common sense observations highlight the challenge facing the Norwegian court that has to decide on Breivik's sanity in the coming weeks. The topic has been the subject of intense media coverage, which increased with reports that a second psychiatric assessment had deemed Breivik sane. This contradicts the initial evaluation by court-appointed psychiatrists which reportedly found that he was suffering from psychosis, rendering him insane. The experts do not agree, and the issue will fall to the court to decide - but decide on what, exactly?

An understanding of the law on insanity is key to understanding what will take place in court. This is because the court is asked to decide not whether Breivik is mentally ill, but rather whether he is legally insane, which is a different question.

In England, the insanity law sets out very specific criteria, the M'Naghten rules, that must be met if a person who is mentally ill is not to be held responsible for his or her actions. To be deemed legally insane, the person, because of a disease of the mind, must not know the nature or quality of the act, or must not know that what he or she was doing was wrong. In real terms, this means that the accused would have to prove to the jury that they did not realize, at the time of the crime, what they were physically doing or did not know that it was against the law. This can be difficult to prove, even for a defendant who has psychotic symptoms which distort their view of reality. If proven, the court finds the defendant not guilty by reason of insanity: they are not held criminally responsible for what they did.

Because of these strict rules, many defendants who have a mental illness do not reach the legal test of insanity, and it is a rare defence in English law. Their mental health problems can be taken into account in other ways, but they are still held at least partially responsible for their crimes. Based on reports of his testimony so far, it appears very unlikely that Breivik's legal sanity would be in question if the English insanity law applied to him.

In Norway, however, the General Civil Penal Code states that 'a person who was psychotic or unconscious at the time of committing the act shall not be liable to a penalty'. This wording, in contrast to the English law, does not make specific reference to the person's knowledge or understanding of their criminal act, but rather asks whether the person was psychotic. This legal definition potentially includes many more people than the English definition, and helps to explains why Breivik's sanity is in question in Norway when it likely wouldn't be in England.

So where does this leave the first of the reasonable views expressed on the Clapham omnibus: that the crime was too well organized and concealed to be carried out by an insane man? In an English court, evidence of concealment and planning could well be used to prove the person did know the meaning of their actions, helping to rule out legal insanity. The same evidence may not necessarily rule out legal insanity in Norway.

And what about the second view: questioning whether any sane person could commit such a crime? We must remember that in the eyes of the law an insane person is someone who, on account of their mental illness, did not know the meaning of their actions, and should not be liable to punishment. If the question of sanity is viewed within these legal terms, the Man on the Clapham Omnibus might then be able to accept that it is entirely possible for a sane person to commit mass murder, and would be grateful that it is not he who has the responsibility of deciding this issue in the case of Anders Behring Breivik.