Snell v Farrell

Snell relaxed the rigorous English approach to causation by accepting common-sense inference where scientific certainty is impossible. The plaintiff bears the legal burden, but the trier of fact can draw a robust inference from the totality of the evidence.

Snell suffered loss of vision in her right eye after her ophthalmologist proceeded with cataract surgery despite a retrobulbar haemorrhage. The medical experts could not say with certainty what caused the vision loss.

How is causation proved in a medical-negligence case where scientific certainty is impossible?

Causation established by common-sense inference. Trial verdict for plaintiff restored.

In medical-negligence cases (and others) where scientific evidence is incomplete, causation can be established by drawing a robust common-sense inference from the totality of the evidence. The legal burden remains on the plaintiff, but the trier of fact may infer causation in the absence of explanation by the defendant.

Sopinka J rejected the notion that causation requires scientific precision. Where the defendant's conduct creates a situation in which causation can readily be inferred, and the defendant offers no satisfactory alternative explanation, the trier of fact may find causation. This is not a reverse onus — but the practical effect is similar where the defendant has unique access to the facts.

Standard authority on the application of but-for in medical and complex causation cases. Reaffirmed in Resurfice and Clements as the dominant Canadian causation framework. Often distinguished from English Bonnington Castings, McGhee, Fairchild line.

Snell v Farrell, [1990] 2 SCR 311, 1990 CanLII 70 (SCC).

Dickson CJ, Lamer J, Wilson J, La Forest J, L'Heureux-Dubé J, Sopinka J, Gonthier J