Why get a solicitor? It's a big question with many possible answers, one of which is this: solicitors know the lingo.

It's not for nothing that the word "legalese" exists. This means "language containing an excessive amount of legal terminology or of legal jargon". If you choose to fight your legal battle without expert advice, you're faced with a dense jungle of concepts and phrases that don't immediately yield to common sense.

Take the idea of a "reasonable person". This shadowy figure pops up in cases of negligence – but who is it? Sometimes known as a "reasonable man" or "the man on the Clapham omnibus", it can play a pivotal role in deciding the outcome of a case.

Put simply: the reasonable person is a character in legal fiction.

Again, the language can be tricky here. By "a character in legal fiction" we don't mean Atticus Finch in To Kill a Mockingbird or Ally McBeal. We mean that the reasonable person is a hypothetical creation of the legal system that's used as a benchmark for reasonable behaviour.

This figure is one of a whole host of fictional characters in law, rubbing shoulders with the "right-thinking member of society", the "officious bystander", the "reasonable parent", the "reasonable landlord" and "the fair-minded and informed observer".

But what is the role of the reasonable person?

In a case of negligence, it needs to be established whether the defendant acted negligently.

So far, so obvious – but how do you define "negligence"? One person's notion of negligence may differ from another's and, within a jury, this could lead to twelve competing views. So over time, the figure of the reasonable person has come to be a yardstick of reasonable behaviour – not an average or typical individual but a person who can be trusted to behave reasonably in any given situation.

The reasonable person, then, is a tool used by legal professionals to set a yardstick for reasonable behaviour.

A brief history of the reasonable person

In English law, it all began with a haystack.

The English case of Vaughan v. Menlove took place in 1837. It centred on a man who had stacked hay in a way that put it at risk of catching fire. He was warned of the danger but didn't act. Eventually, the hay ignited and burned his barns before spreading to his landlord's nearby cottages.

Menlove's attorney argued that his client had the "misfortune of not possessing the highest order of intelligence" and that he had acted to the best of his abilities.

If Menlove had acted according to his best judgement, could he be found guilty of negligence? This would be a subjective interpretation of negligence, where the nature of negligence changes according to the character of the person with a duty of care.

This is where the reasonable person (then "reasonable man") comes in. This figure acts as an objective guide to questions of care and responsibility. Menlove may have acted in the only way he could, but he didn't exercise a "caution such as a man of ordinary prudence would observe" – and this, the judge decided, was the clincher.

The phrase surfaced again in 1856 in Blyth v. Company Proprietors of the Birmingham Water Works. In this case, the waterworks company was sued by a citizen named Mr Blyth for installing a bit of hardware called a fireplug that failed, causing flood damage to his house.

The court decided that the waterworks company hadn't been negligent because they did what a "reasonable person" would have done. They couldn't be held accountable for the unexpectedly severe frost that caused the fireplug to fail. As the defendant's lawyer said:

"Negligence is the omission to do something which a reasonable man, guided upon those considerations which ordinarily regulate the conduct of human affairs, would do, or doing something which a prudent and reasonable man would not do."

As with Menlove, the individual characteristics of the defendant aren't important. What matters is the objective standard set by the reasonable person.

Are there any exceptions?

By now you might be wondering how this legal fiction can be used across the board when the personal circumstances of defendants can vary so dramatically.

In practice, courts acknowledge that sometimes the reasonable person needs to have the same limitations as the defendant. A clear example is that a defendant with a disability is judged according to how a reasonable person with the same disability would act. And children under the age of 10 aren't tried at all because they aren't seen to have the capacity to behave as a reasonable person should.

In other instances, however, the limitations of the defendant aren't held to be relevant. Here are a few examples:

1. People with a mental illness

Controversially, the concept of the reasonable person doesn't make allowances for mental illness, and insanity isn't a defence.

2. Professionals

If you perform an act that belongs to the professional sphere, you're not judged against the standard of the reasonable person but that of the reasonable professional – and this applies whether or not you are a trained professional.

3. People who lack experience

The actual level of experience of a defendant isn't allowable as a defence. If Menlove had never stacked hay before, that wouldn't be considered relevant. Instead, he would be held to the standard of our reasonable person, who has the relevant experience to behave reasonably.

If you're a victim of negligence, you'll more than likely come across the concept of the reasonable person. We hope this guide has helped make sense of it, but remember – law is a complex field, and your best bet is to seek expert advice so you go in well-equipped with the facts.

Have you suffered because of professional negligence or medical malpractice? Our negligence solicitors are ready with plain-speaking, no-nonsense advice at a sensible price. To find out more, please don't hesitate to get in touch.