CanadaWhat is the but-for test for proving causation in a personal injury case?
The but-for test asks whether the injury would not have occurred 'but for' the defendant’s negligent act — if the harm would still have happened without the negligence, there is no legal causation.
What the Law Says
Canadian tort law uses the 'but-for' test to determine whether a defendant’s negligence factually caused the plaintiff’s injury. This is a threshold test of factual causation — it does not address fairness or policy, only whether the harm would have occurred without the wrongful act.
The but-for test is not codified in a single statute across Canada but is a common law principle applied uniformly by courts in negligence cases. It forms part of the foundational framework for establishing liability in personal injury claims under provincial and federal tort law.
Courts apply the test as a practical, common-sense inquiry: 'But for the defendant’s negligent conduct, would the plaintiff’s injury have occurred?' If the answer is 'no', factual causation is established. If 'yes', the chain of causation fails at this first step.
What Courts Have Said
The Supreme Court of Canada affirmed and clarified the but-for test in Athey v. Leonati, emphasizing its centrality while addressing complexities like pre-existing conditions and multiple causes.
The Court held that the but-for test remains the standard for factual causation; a defendant is liable for the full extent of injury even if a pre-existing vulnerability (e.g., thin skull) contributed — but not if the injury would have occurred anyway (e.g., crumbling skull).
What to Do
Identify the specific negligent act or omission alleged against the defendant.
Ask: 'Would the injury have occurred *but for* that act?' Use medical evidence, timelines, and expert testimony to assess.
If multiple causes exist, determine whether the defendant’s conduct was a 'material contribution' — though but-for remains primary unless exceptional circumstances apply (per Athey).
Distinguish between 'thin skull' (defendant takes victim as found — full liability) and 'crumbling skull' (injury would have occurred regardless — limited or no liability).
Sources
Not legal advice. This article is general information based on publicly available sources, written for educational purposes. Laws change and individual situations vary. Consult a licensed attorney in your jurisdiction before acting on anything you read here. Last reviewed: 2026-06-08.