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

I also refer to Athey v. Leonati, 1996 CanLII 183 (SCC), [1996] 3 S.C.R. 458 (S.C.C.) where Major J. explained, at 467, that it is not necessary to establish that the defendant’s negligence is 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 the 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 in liability because of the existence of other preconditions: defendants remain liable for all injuries caused or contributed to by their negligence [emphasis in original].