To establish liability at common law, a plaintiff must first establish that a defendant owed a duty of care (a responsibility) towards the plaintiff or the deceased person. Over a period of years the law has established a requirement that people (including companies and governments) should conduct their affairs to a standard to allow for the protection of others against unreasonable risks. The standard required is that of the reasonable person. What a reasonable person would have done in the circumstances is a matter for the courts to decide, after considering all the facts surrounding the incident or incidents which caused injury to the plaintiff or the deceased person. This involves asking questions like:

The courts have considered these problems in some detail over the years. The principles applied in specific areas are discussed in the following chapters.

If the defendant fails to do something which a reasonable person would do, or alternatively does something which a reasonable person would not do, the law states that there has been negligence. This does not necessarily mean that the plaintiff is entitled to damages. The plaintiff must show that there has been an actual injury. For example, a plaintiff who has been exposed to dangerous chemicals as a result of the negligence of another must show an actual injury, not just the mere possibility of injury.

The plaintiff must also show that there is a reasonably close connection between the defendant's conduct and the injuries suffered by the plaintiff or the deceased person. Either the damage must not be too remote from the wrong conduct, or the wrong conduct must be an effective cause of the injuries. However, the wrong conduct need not be the only cause of injury.

The courts must consider whether the plaintiff or the deceased person engaged in conduct that will either defeat the action for damages completely or reduce the amount of damages recovered.

The defence of volenti non fit injuria (consent to the injury) is applicable if the defendant can prove that the plaintiff or the deceased person had full knowledge of the risk of injury to themselves and had voluntarily assumed or undertaken the liability resulting from that risk. For example, a plaintiff may have been injured in a collision while travelling in a car with a driver known to be drunk. If this defence succeeds, it completely defeats the plaintiff's claim for damages, even if the defendant was negligent as well.

If the defendant can show that the negligence of the plaintiff or the deceased person also contributed to the damage sustained, this will only reduce the plaintiff's claim for damages by the percentage contribution found by a Judge or jury. For example, if the court finds that the plaintiff or the deceased person was 10% to blame for their own injuries because of not wearing a seat belt at the time of the collision, the amount of damages payable to the plaintiff will be reduced by 10%.

In deciding whether a defendant has been negligent, a Judge or a jury may have to consider whether there has been a breach of a statutory duty. For example, a person may have been injured in a collision where the defendant had failed to give way to the right in accordance with the Road Traffic Regulations. Such a breach is at least evidence of negligence, as there has been a breach of the common law duty to take care. Courts have also used a breach of a statutory duty as being negligence in itself, particularly where the statute does not provide criminal penalties for the breach of that statute.

Note: These principles have now been significantly altered by legislation, and in particular by the Wrongs and Other Acts (Law of Negligence) Act 2003 (see: "Wrongs Act", below).