What is the "but for" test for causation in motor vehicle accidents?

As I will explain, the trial judge appears to have misinterpreted the leading case, Clements v. Clements, 2012 SCC 32, [2012] 2 S.C.R. 181, and what is meant by the “but for” and “material contribution” tests for causation. It appears that she decided that she had to apply the “material contribution” test because there were multiple actors (including the respondents), whose conduct was at issue, and she may well have assumed that the “but for” test required the identification of a single negligent act that was the sole cause of the respondents’ loss or injury. When the trial judge spoke of each appellant’s “material contribution to the [respondents’] loss and injury”, she was in fact applying the “but for” test. The trial judge made certain findings of fact that are consistent with “but for” causation in this case, and her reasons, when read fairly and in their entirety, do not preclude, but rather support, a finding against all of the appellants of “but for” causation. (i) “But for” is the correct test for causation in this case