The "but for" test is the standard method for proving causation in negligence

[White Wolf]

The "but for" test is the standard method for proving causation in negligence, requiring a plaintiff to show that the injury would not have occurred but for the defendant's negligence. If the injury would have happened regardless of the defendant's actions, then the defendant is not liable. This test is applied on a balance of probabilities, meaning the plaintiff must show it is more likely than not that the negligence caused the harm.

How the "but for" test works
The "what if" question: The test uses "what-if" thinking to establish a factual causal link. It asks: If the defendant had not been negligent, would the plaintiff still have suffered the injury?
Defendant's conduct must be a necessary factor: If the answer to the "what if" question is that the injury still would have happened, the "but for" test is not met. The defendant's negligence must be a necessary condition for the injury to have occurred.

Example: In Barnett v. Chelsea & Kensington Hospital, a man died from arsenic poisoning. The hospital had turned him away, but the court found that even if he had been admitted, he likely would have died. Therefore, the hospital's negligence was not the "but for" cause of death.

Not the sole cause: The defendant's negligence does not have to be the only cause of the injury. It is enough if it is one of the causes that materially contributed to the harm, even if other non-tortious factors were also present.

When the "but for" test may be insufficient
In rare cases where it's impossible to prove causation using the "but for" test (such as when multiple negligent parties are involved and it's unclear who caused the injury), courts may use an alternative test called the material contribution to risk of injury.