A negligently starts a brushfire that spreads toward B's barn. At the same time, a lightning strike ignites a second fire nearby. The two fires merge before reaching the barn, and the merged fire completely destroys it. Either fire, acting alone, would have been sufficient to cause the destruction. If B sues A for negligence, which of the following best states A's liability for the loss?
A is not liable because the lightning fire was a superseding cause that broke the chain of causation.
A is not liable unless B can prove the barn would not have been destroyed without A's fire.
A is not liable because his negligence was not a but-for cause once the lightning-started fire merged with his.
A is liable because his negligence was a substantial factor in causing the destruction of the barn.
When two independent forces are concurrently operating and each alone would have produced the harm, each negligent force is treated as a substantial factor under the Restatement (Second) of Torts § 432(2). Because A's negligence was a substantial factor, A is liable-even though the lightning-caused fire was also sufficient. The lightning was not a superseding cause because it did not break the causal chain; both causes operated concurrently. A need not escape liability simply because the harm would have occurred without his negligence. Options pointing to lack of but-for causation or superseding cause are incorrect, and requiring B to prove that A's fire was the sole cause misstates the substantial-factor rule.
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