How is the assessment of damages when there is an intervening event or pre-existing condition?

In Athey v. Leonati, 1996 CanLII 183 (SCC), [1996] 3 S.C.R. 458 [Athey], Major J. expressed the issue of the assessment of damages in cases where there is an intervening event or, for that matter, a pre-existing condition, in the following terms at para. 32: The essential purpose and most basic principle of tort law is that the plaintiff must be placed in the position he or she would have been in absent the defendant's negligence (the "original position"). However, the plaintiff is not to be placed in a position better than his or her original one. It is therefore necessary not only to determine the plaintiff's position after the tort but also to assess what the "original position" would have been. It is the difference between these [two] positions, the "original position" and the "injured position", which is the plaintiff's loss. In the cases referred to above, the intervening event was unrelated to the tort and therefore affected the plaintiff's "original position". The net loss was therefore not as great as it might have otherwise seemed, so damages were reduced to reflect this.