A necessary cause; a cause without which the effect in question could not have happened.

Explanation

The "but for" test of causation — if the injury would not have occurred without the defendant's act, that act is a necessary cause and liability may attach.

Where this applies

• A patient sues her doctor for a botched surgery — she must show that 'but for' the doctor's error, the injury would not have occurred.
• A factory dumps chemicals upstream and fish die downstream — the fishermen must prove the contamination was a necessary cause of the fish kill, meaning the fish would not have died without it.
• A pedestrian is hit by a car that ran a red light — the driver's violation is the causa sine qua non because the collision would not have happened if the driver had stopped.