Where a defendant's negligence exacerbates a pre-existing condition, the evidentiary presumption in Purkess v Crittenden throws upon the defendant only an evidentiary burden of disentangling causes, not a legal onus of proof. The tortfeasor remains responsible only for the damage caused by the tort, and damages must account for the probability that some or all of the damage would have occurred in any event. A trial judge who treats the Purkess evidentiary onus as a legal onus and makes no allowance for pre-existing conditions commits appealable error.
The full text is available to signed-in members, including the 9 later cases that cite this judgment.
2 of the 9 citing cases carry a classified treatment. How each court treated it is available to signed-in members.