What is the test for determining whether there is a but for?

The appellants argue that the trial judge resorted to the “material contribution” test for causation, as she purported to do, because she recognized that she was unable to make the necessary factual findings for “but for” causation. They point to the absence of evidence about the specific condition of the tank in 2006 and 2007 and the timing of when water began to accumulate in the tank and the progress of the corrosive process. They refer to Gendron v. Thompson Fuels, 2017 ONSC 4009, aff’d 2019 ONCA 293, in which there was evidence with respect to the rate of corrosion leading to a discharge from a fuel oil tank, and they say that the absence of this type of evidence in this case was fatal to any finding of “but for” causation.