Whether the defendant's negligence was a cause-in-fact of the plaintiff's harm is a concrete, scientific question for which there is at least theoretically always a correct "yes or no" answer. Why is there always a "right answer" in the abstract regarding whether a defendant's negligence was a cause-in-fact of the plaintiff's harm? Given that this is so, why do you think establishing cause-in-fact is often a difficult issue in negligence lawsuits