This week, the key to a locker that contained the binoculars for the lookout on the Titanic went up for auction. The key could have saved 1,522 lives but was accidentally left behind when the ship embarked on its maiden voyage in 1912. Fred Fleet, a surviving lookout, was asked what difference the binoculars might have made. "Enough to get out of the way," he said.

Quite often, a small mistake can have severe consequences. In order to determine how large a compensation bill a defendant must pick up, it is important in law to have rules about how far one thing can be said to have caused a certain effect. These are known as the rules of causation. They vary according to which branch of law applies, such as the civil law of torts, including negligence.

The basic test used for deciding whether one thing caused another is called the “but for” test. But for Emily having spilt oil on the floor, Charlotte wouldn’t have slipped and hurt herself. However, the law uses a sort of double test: it isn’t enough just to show a factual link between a cause and its alleged effect. You also have to show that a cause was a direct or material reason why something happened.

Sometimes under the law of negligence a claimant is denied damages because, although the defendant did do something negligent, the chain of causation between that negligence and the injury is broken by a novus actus interveniens – a new intervening act. In a case in 1915, a brewery was sued after its lorry, full of beer barrels, crashed into a shop. The lorry driver had left it unattended for a brief moment outside a pub in Portsmouth when a couple of soldiers tried unsuccessfully to drive it away. The court decided that even if the brewery had been negligent in leaving the lorry unattended, what happened next was unpredictable, a new intervening act.

By contrast, in a case in Birmingham in 1948, a decorator left the house where he was working to get some more wallpaper but, as the owners were out, he left the front door on the latch so he could get back in. During the two hours he was gone, expensive jewellery was stolen from the house. The court decided that the burglary was not a new intervening act – it was a predictable consequence of the decorator’s negligence and he should be liable for the loss.

Common sense is an important guide in applying the principles of causation. In a case in 1962, a man negligently collided with a Rolls Royce in London. The same part of the car - the wing and bumper - had been previously hit by another driver, and had not been repaired. The first driver hadn't paid up, so the owner sued the second. But the court decided that the second driver didn't have to pay for the repairs because the wing was already damaged. In another case, it was decided that a doctor's negligence in failing to give proper treatment to a man suffering from arsenic poisoning was not the cause of the man's death as the poison was so far advanced by the time he reached hospital that even proper treatment could not have saved him.

Another aspect of the law on causation is that you must take your victim "as you find them". In other words, just because the victim suffers more harm than another person may have in the same situation doesn't make you are any less liable. In 1950, a worker at the Glaucus Iron Works in Poplar was splashed with molten metal on his lip. It triggered a pre-malignant condition, he developed cancer, then he died. The court ruled that since the type of injury he suffered was a reasonably foreseeable result of negligence on the company's part, it was liable to pay compensation for the man's death. It didn’t matter that the company couldn’t reasonably have known that he had a pre-existing condition that the splash could aggravate.

In the case of the locker key for the Titanic's binoculars, it is difficult, legally speaking, to establish a convincing link between the act of forgetting it and the death of 1,522 people. The forgetfulness wasn’t a material reason for the disaster. There were many new intervening acts between it and the final catastrophe, such as the decisions not to break open the locker to get the binoculars, not to get alternative binoculars and not to abort the voyage if binoculars were critically important. Not to mention that most of the deaths would have been avoided if there had been enough lifeboats.

The common law in this area, as in others, can give the impression that judges made it up as they went along. That’s because they did. As Sir Thomas Holland described it in the 19th-century, the common law is “chaos with a full index”. Certainly, the rules on causation are many and varied. But deciding whether a defendant’s conduct is a legal cause of some injury or loss is a matter of sense more than science.

In causation cases, an important silent question helps courts determine the outcome: would it be fair to make the defendant, and people like him in future, pay for what has happened?

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

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