What is the test for determining causation in negligence?

A cause must be necessary to bring about the injury or loss before liability attaches. The standard test for causation in negligence is the “but-for” test, which questions whether the injury or loss would have occurred had the alleged negligence not occurred. This means the negligent act must have been necessary to bring about the injury in that the injury would not have occurred without it: Ediger v. Johnston, 2013 SCC 18 at para. 28. A finding of causation, once made by the trial judge, is a finding of fact: Ediger at para. 29.