For a further discussion of the logic of redundant causation, correcting an omission in the text, see this virtual footnote (added after the book was printed, hence not shown by an icon on the hardcopy). 

The actual cases of redundant causation that I know of involve liability for damage due to fires started by two independent causes--raising the question of whether each party has a defense in the fact that the fire would have happened anyway. In some both parties are human, in some one fire is of unknown origin, and in some one fire is believed due to natural causes. From one such case:

"We, therefore, have this situation: The northeast fire was set by sparks emitted from defendant's locomotive. This fire, according to the finding of the jury, constituted a proximate cause of the destruction of plaintiff's property. This finding we find to be well supported by the evidence. We have the northwest fire, of unknown origin. This fire, according to the finding of the jury, also constituted a proximate cause of the destruction of the plaintiff's property. This finding we also find to be well supported by the evidence. We have a union of these two fires 940 feet north of plaintiff's property, from which point the united fire bore down upon and destroyed the property. We, therefore, have two separate, independent, and distinct agencies, each of which constituted the proximate cause of plaintiff's damage, and either of which, in the absence of the other, would have accomplished such result.

It is settled in the law of negligence that any one of two or more joint tort-feasors, or one of two or more wrongdoers whose concurring acts of negligence result in injury, are each individually responsible for the entire damage resulting from their joint or concurrent acts of negligence. This rule also obtains--

"where two causes, each attributable to the negligence of a responsible person, concur in producing an injury to another, either of which causes would produce it regardless of the other, * * * because, whether the concurrence be intentional, actual or constructive, each wrongdoer, in effect, adopts the conduct of his coactor, and for the further reason that it is impossible to apportion the damage or to say that either perpetrated any distinct injury that can be separated from the whole. The whole loss must necessarily be considered and treated as an entirety." Cook v. Minneapolis, St. Paul & Sault Ste. Marie R. Co.,98 Wis. 634 , at page 642, 74 N. W. 561 , 566 (40 L. R. A. 457 , 67 Am. St. Rep. 830 ).

That case presented a situation very similar to this. One fire, originating by sparks emitted from a locomotive, united with another fire of unknown origin and consumed plaintiff's property. There was nothing to indicate that the fire of unknown origin was not set by some human agency. The evidence in the case merely failed to identify the agency. In that case it was held that the railroad company which set one fire was not responsible for the damage committed by the united fires because the origin of the other fire was not identified. In that case a rule of law was announced, which is stated in the syllabus prepared by the writer of the opinion as follows: "A fire started by defendant's negligence, after spreading one mile and a quarter to the northeast, near plaintiffs' property, met a fire having no responsible origin, coming from the northwest. After the union, fire swept on from the northwest to and into plaintiffs' property, causing its destruction. Either fire, if the other had not existed, would have reached the property and caused its destruction at the same time. Held:

(1) That the rule of liability in case of joint wrongdoers does not apply.

(2) That the independent fire from the northwest became a superseding cause so that the destruction of the property could not, with reasonable certainty, be attributed in whole or in part to the fire having a responsible origin; that the chain of responsible causation was so broken by the fire from the northwest that the negligent fire, if it reached the property at all, was a remote and not the proximate cause of the loss."

Emphasis is placed upon the fact, especially in the opinion, that one fire had "no responsible origin." ... From our present consideration of the subject, we are not disposed to criticise the doctrine which exempts from liability a wrongdoer who sets a fire which unites with a fire originating from natural causes, such as lightning, not attributable to any human agency, resulting in damage. ...

Speaking of the decision in the Cook Case, Thompson, in his work on Negligence, § 739, says:

"The conclusion is so clearly wrong as not to deserve discussion. It is just as though two wrongdoers, not acting in concert, or simultaneously, fire shots from different directions at the same person, each shot inflicting a mortal wound. Either wound being sufficient to cause death, it would be a childish casuistry that would engage in a debate as to which of the wrongdoers was innocent on the ground that the other was guilty."

Owen, J. in Kingston v. Chicago & N.W. Ry. 191 Wis. 610 , 211 N.W. 913 (1927)

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Judge Owen raises the question of whether it matters if one of the two fires is of natural origin but never adequately answers it. A partial answer in the general case of redundant causation is suggested by the discussion in Chapter 14. One reason not to hold the two hunters innocent is that doing so may be an invitation to conspiracy to murder--via an arranged "accident"--which isn't a problem if one of the causes is natural. But that argument does not seem relevant to the fire cases.

The chief argument for liability in the two hunter case is that, because it is costly to adequately punish hunters who do cause death, we supplement the ex post punishment for causing death with an ex ante punishment for actions that might cause death--the same argument earlier used to explain punishment of attempts.

But that argument too seems irrelevant to the fire cases; presumably the Chicago & Northwestern Railway had sufficient resources to fully compensate the plaintiff for the loss of his property. So holding the railroad guilty only when its negligence results in the loss of the property will provide an adequate level of deterrence. Hence the court was wrong. Tortfeasors who are unlikely to be judgement proof, in clear cases of redundant causation, where there is no risk of conspiracy, ought not to be held liable--whether the two agencies were human or one of them natural.