Abe is camping and builds a campfire. Abe fails to properly put the fire out, and it starts a forest fire. Abe's fire merges with another forest fire, and the combined fire burns down Valerie's house. The fire marshall testifies that either fire was sufficient to burn down the house even if it had been the only fire. Abe argues that Valerie's house would have burnt down anyway, so he shouldn't be held responsible. Should Abe be liable for the destruction of Valerie"s house?

Does it matter if the other fire was started by a lightning strike or by Boris--who also was camping and also failed to put out his campfire?