If you study law, sooner or later you will come across the issue of causation. That is, a defendant should only be liable for damages that he caused the plaintiff.
For US law students I think the first time they typically encounter causation issues is in torts when studying negligence. Every student learns that a plaintiff in a negligence lawsuit typically must prove that (i) defendant owed the plaintiff a duty of care; (ii) defendant breached his duty of care; (iii) causing; (iv) injury to the plaintiff.
How do we know whether a defendant’s breach caused the injury? Two types of tests you will commonly see in the US are the “but for” and the “substantial factor” tests.
The “but for” test asks, “Would the plaintiff have suffered the injury if defendant hadn’t acted carelessly?”
This is a fairly obvious question. Let’s say the defendant drops a banana peel on his home’s entranceway and leaves it there. The plaintiff comes by and slips on the peel. If the defendant hadn’t left the peel there the plaintiff would not have tripped so we can say that the defendant’s sloppiness was the “but for” cause of plaintiff’s injury. Done!
Sometimes a plaintiff would have gotten damaged even if the defendant did not act carelessly but we still want to hold the defendant responsible. The classic US case is where a defendant causes one fire, the weather or another defendant causes another fire, and the plaintiff loses his house in one giant fire when the two fires converge.
The plaintiff in this case has a “but for” causation problem. Even if defendant didn’t start a fire the house would have gotten burned by the other fire. How can we be sure that the defendant’s fire destroyed the house? In these cases, courts might apply the substantial factor test and ask whether the defendant’s fire was a “substantial” factor in causing the damage to the plaintiff’s house. There may be other tests that a court will apply but the substantial factor test is the most common.
The substantial factor test is important in toxic injury cases. For example, if a defendant works in a factory and develops cancer, he might allege that the cancer resulted from asbestos poisoning. The defendant factory owner will likely question whether the factory’s asbestos was a substantial factor in causing the cancer or whether other factors played a far more significant role.
Below is a video discussing but for and substantial factor causation.