What is the test for establishing that negligence was not the sole cause of injury?

In Athey v. Leonati, 1996 CanLII 183 (SCC), [1996] 3 S.C.R. 458, Major J. set out the general principles of causation at paras. 17-19: It is not now necessary, nor has it ever been, for the plaintiff to establish that the defendant's negligence was the sole cause of the injury. There will frequently be a myriad of other background events which were necessary preconditions to the injury occurring. ... As long as a defendant is part of the cause of an injury, the defendant is liable, even though his act alone was not enough to create the injury. There is no basis for a reduction of liability because of the existence of other preconditions: defendants remain liable for all injuries caused or contributed to by their negligence. … The law does not excuse a defendant from liability merely because other causal factors for which he is not responsible also helped produce the harm ....