Once the plaintiff has shown that the defendant behaved negligently, he must then show that this behavior “caused” the injury complained of. Actually, P must make two quite distinct showings of causation:
A. General significance: When the plaintiff alleges that the defendant “caused” his injuries, it is pretty clear what he means – that the injuries were the actual, factual, result of the defendant’s actions. In most cases, the question of “causation in fact” is a purely factual one, with few legal or policy issues attached to it.
1. “But for” test: In the vast majority of situations, the defendant’s conduct is the (or a) cause of the plaintiff’s injuries if it can be said that “Had the defendant not so conducted herself, the plaintiff’s injuries would not have resulted.” This formulation is sometimes known as the “sine qua non” or “but for” test.
a. Third Restatement’s formulation: The Third Restatement implements the idea of a “but for” cause. After saying that liability will exist only where the defendant’s tortious conduct was a “factual cause” of the plaintiff’s injury, the Restatement goes on to express the idea that a “but for” cause is always a factual cause: “Conduct is a factual cause of harm when the harm would not have occurred absent the conduct.” Rest. 3d (Liab. for Phys. & Emot. Harm) §26.