What is the test for determining whether Petro Tag is a contributory factor in a Co-op’s loss of income?

Conduct attributed to Petro Tag must be the cause of the Co-op’s loss or there is no liability (Canadian Tort Law, supra, p. 115). Generally, the test for causation is the “but for” test. In other words, the Co-op must show that the damage would not have occurred but for the negligence of Petro Tag. (Athey v. Leonati, 1996 CanLII 183 (SCC), [1996] 3 S.C.R. 458 at para. 14) The Co-op does not have to establish that the actions of Petro Tag were the sole cause of the damage: 17 ...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. (Athey v. Leonati, supra, at para. 17)