Foreseeability.
Legal Causation is usually expressed as a question of 'foreseeability'. An actor is liable for the foreseeable, but not the unforeseeable, consequences of his or her act. For example, it is foreseeable that if I shoot someone on a beach and they are immobilized, they may drown in a rising tide rather than from the trauma of the gunshot wound or from loss of blood. However, it is not (generally speaking) foreseeable that they will be struck by lightning and killed by that event.
This type of causal foreseeability is to be distinguished from foreseeability of extent or kind of injury, which is a question of remoteness of damage, not causation. For example, if I conduct welding work on a dock that lights an oil slick that destroys a ship a long way down the river, it would be hard to construe my negligence as anything other than causal of the ship's damage. There is no novus actus interveniens. However, I may not be held liable if that damage is not of a type foreseeable as arising from my negligence. That is a question of public policy, and not one of causation.
Example.
An example of how foreseeability does not apply to the extent of an injury is the eggshell skull rule. If Neal punches Matt in the jaw, it is foreseeable that Matt will suffer a bodily injury that he will need to go to the hospital for. However, if his jaw is very weak, and his jaw is dislocated by the punch, then the medical bills, which would have been about $5,000 for wiring his jaw shut had now become $100,000 for a full-blown jaw re-attachment. Neal would still be liable for the entire $100,000, even though $95,000 of those damages were not reasonably foreseeable.
Other relevant considerations.
Because causation in the law is a complex amalgam of fact and policy, other doctrines are also important, such as foreseeability and risk. Particularly in the United States, where the doctrine of 'proximate cause' effectively amalgamates the two-stage factual legal causation inquiry favored in the English system, one must always be alert to these considerations in assessing the postulated relationship between two events.
