What is the test for determining whether past events such as negligence and causation must be proved on a balance of probabilities?

Athey v. Leonati, 1996 CanLII 183 (SCC), [1996] 3 S.C.R. 458, relied on by the plaintiff, held that past events such as negligence and causation between fault and injury must be proved on a balance of probabilities and thereafter treated as certainties. However, Mr. Justice Major stated (at para. 27) that hypothetical events need not be proved on a balance of probabilities, and they are simply to be given weight according to their relative likelihood. In assessing hypothetical events there is no reason to distinguish between those before trial and those after trial. In making an allowance for contingencies the trial judge was assessing the hypothetical events that could have effected [sic] the plaintiff’s employment earnings, according to the assessment to their relative likelihood.