What is the 'but for' test for causation in an Ontario negligence claim?

The "but for" test is how Ontario courts usually decide whether a defendant's careless conduct actually caused your injury, as opposed to merely happening alongside it. The question is simple to state: but for the defendant's negligent act or omission, would the injury have happened anyway? If the injury would have occurred regardless of what the defendant did, causation is not made out, even if the defendant was careless in some general sense. If the injury would not have happened without the negligence, the test is satisfied, on a balance of probabilities rather than certainty.

The test becomes harder to apply cleanly where there are multiple possible causes acting together, or where medical or technical evidence cannot say with confidence what would have happened in a hypothetical scenario without the negligence. In narrow situations like these, courts have allowed a more flexible, common-sense approach to causation rather than a strict but-for analysis, though this remains the exception rather than the rule. In practice, causation is often the hardest part of a negligence claim to prove, because it usually depends on expert medical or technical opinion about what would, or would not, have happened otherwise, so early and well-documented evidence matters.