What is the difference between the rules of damage assessment in tort and the rule of causation in tort?

In Blackwater v. Plint, 2005 SCC 58, McLachlin C.J.C., at para. 78, discussed the difference between causation as the source of the loss and the rules of damage assessment in tort. She said: … The rules of causation consider generally whether “but for” the defendant’s acts, the plaintiff’s damages would have been incurred on a balance of probabilities. Even though there may be several tortious and non-tortious causes of injury, so long as the defendant’s act is a cause of the plaintiff’s damage, the defendant is fully liable for that damage. The rules of damages then consider what the original position of the plaintiff would have been. The governing principle is that the defendant need not put the plaintiff in a better position than his original position and should not compensate the plaintiff for any damages he would have suffered anyway: Athey.