(dissenting).
I respectfully dissent. The current pattern jury instruction on pre-existing conditions misstates Minnesota law and amounts to an improper extension of the single indivisible injury rule. Because the jury instruction embraced by plaintiffs’ counsel before the jury and given in this single plaintiffisingle tortfeasor case permitted the jury to allocate liability for harm that may not have been caused by the tortious conduct, I would affirm the remand for a new trial.
In Minnesota, a person who has a preexisting condition is entitled to recover damages for an aggravation of that condition, “recovery being limited, however, to *163the additional injury over and above the consequences which normally would have followed from the preexisting condition absent defendant’s negligence.” Schore v. Mueller; 290 Minn. 186, 189, 186 N.W.2d 699, 701 (1971) (citing Nelson v. Twin City Motor Bus Co., 239 Minn. 276, 280, 58 N.W.2d 561, 563 (1953)). “Put another way, damages for aggravation of a preexisting condition are simply a means to assure that the defendant pays only for the harm he causes, not the harm plaintiff already had.” Leubner v. Sterner, 493 N.W.2d 119, 122 (Minn.1992) (citing Schore, 290 Minn, at 189, 186 N.W.2d at 701, and Bloomquist v. Minneapolis Furniture Co., 112 Minn. 143, 148, 127 N.W. 481, 482 (1910)).
The burden has long been upon the plaintiff to allocate harm caused by the defendant “over and above” the original condition. Watson v. Rheinderknecht, 82 Minn. 235, 238, 84 N.W. 798, 799 (1901) (burden was upon the plaintiff “to show in what respect, and to what extent, his present condition could be attributed to the assault and battery, and what could be more properly established as the result of his army experience.”). For well over 30 years our pattern jury instruction on items of damage for any aggravation of a preexisting condition has reflected the limitation on the measure of damages to the additional injury caused by the aggravation. 4 Minnesota District Judges Association, Minnesota Practice, Jury Instruction Guides — Civil, JIG 163 (3d ed.1986).1 Our rules have been consistent with the Restatement of Torts rules for allocating between harm resulting from a pre-exist-ing condition for which the defendant is in no way responsible and the further harm which his tortious conduct caused. Restatement (Second) of Torts § 433A (l)(b) cmt. e (1965); see also Restatement (Third) of Torts: Apportionment of Liability § 26 cmt. f (2000) (“As long as any person caused only a part of damages, * * * the damages are divisible, irrespective of the timing.”).
The current pattern jury instruction on pre-existing conditions represents a significant departure from these rules in that it permits an allocation of liability to the defendant for a plaintiffs pre-existing condition if the jury is unable to determine what part of the plaintiffs injuries predated the accident. 4A Minnesota District Judges Association, Minnesota Practice, Jury Instruction Guides Civil, JIG 91.40 (4th ed.1999).2 The commentary to CIVJIG 91.40 suggests its derivation from Canada by Bandy v. McCarthy, 567 *164N.W.2d 496 (Minn.1997). Canada, however, . involved a unique application of the single indivisible injury rule in which joint and several liability is imposed upon multiple tortfeasors. Canada, 567 N.W.2d at 507-08 (citing Mathews v. Mills, 288 Minn. 16, 20-21, 178 N.W.2d 841, 844 (1970)).3
The single indivisible injury rule can be traced back to Flaherty v. Northern Pac. Ry. Co., 39 Minn. 328, 40 N.W. 160 (1888), and holds that tortfeasors whose separate negligent acts operate together to cause damage to another are each liable for the whole amount of the resulting damage. Id. at 329, 40 N.W. at 160-61. A “necessary corollary” to the rule of holding each defendant liable for the entire harm “is that when the harm can be apportioned on some rational basis, then liability should be proportionate only.” Gerald W. Boston, Apportionment of Harm in Tort Law: A Proposed Restatement, 21 U. Dayton L.Rev. 267, 284 (1996). According to Professor Boston, “by the 1930s, apportionment principles were applied when the plaintiff and the defendant caused similar harm [to plaintiffs property], when the harm was aggravated by acts of nature, and when the defendant aggravated the plaintiffs preexisting harm.” Id. at 285.
Apportionment principles were eventually incorporated in the Restatement (Second) of Torts §§ 433A, 433B (1965). Section 433A provides for the apportionment of harm to causes:
(1) Damages for harm are to be apportioned among two or more causes where
(a) there are distinct harms, or
(b) there is a reasonable basis for determining the contribution of each cause to a single harm.
(2) Damages for any other harm cannot be apportioned among two or more causes.
Restatement (Second) of Torts § 433A (1965).
Pursuant to section 433A, apportionment principles apply to contributing causes of the harms and divisible harms. Included in the divisible harms are pre-existing conditions. Restatement (Second) of Torts § 433A (l)(b) cmt. e (1965). “In the illustrations of the Restatement (Second), the touchstone of apportionment is reliance on the contribution that causes the ultimate harm and not to some actual division of harm itself.” Boston, supra, at 301. The single indivisible injury rule holding multiple tortfeasors jointly and severally liable was incorporated in subsection (2). Restatement (Second) of Torts § 433A (2) cmts. h & i (1965). Comment “h” allows joint and several liability in cases of insolvency or death of one of the tortfeasors and comment “i” applies joint and several principles in cases of an “innocent” cause and two or more culpable causes. Id.
Section 433B of the Restatement (Second) addresses the burden of proof and failure to produce evidence justifying apportionment. Restatement (Second) of Torts § 433B cmt. g (1965). Section 433B (1) states that the plaintiff must prove that the defendant’s tortious conduct caused the harm that is subject to potential apportionment. Section 433B (2) says that it is an exception to the rule stated in subsection (1) and provides for burden-shifting in *165two situations involving multiple tortfea-sors. When the tortious conduct of two or more defendants has combined to bring about the harm, the defendant seeking to limit liability has the burden as to apportionment; and when the plaintiff sues two or more tortfeasors and proves that at least one of them has caused harm but there is uncertainty as to which one has caused it, the burden is on the defendant to prove that he had not caused the harm. Restatement (Second) of Torts § 433B (2) (1965). Comment “c,” in reiterating that subsection (2) is an exception to the general rule that the plaintiff has the burden of establishing that the defendant’s tortious conduct caused the harm, suggests that burden-shifting only applies to multiple tortfeasor situations.
The Restatement (Third) of Torts, generally considered a repudiation of joint and several liability,4 provides for apportionment among two or more persons who caused divisible damages. Restatement (Third) of Torts: Apportionment of Liability § 26 cmt. h (2000). That part of the proposed draft of Restatement (Third) addressing the burden of proof takes no position as to who should bear the burden in apportioning harm between tortious conduct and a pre-existing condition, noting that courts are split on the burden-shifting issue but also recommending that no matter how the burden is allocated, “[s]o long as there is some modicum of evidence that would permit the factfinder to make a causal apportionment, that course is preferable to making whichever party bears the burden of proof to bear the entirety of the loss.” Restatement (Third) of Torts: Liability for Physical Harm § 28 cmt. d (Tentative Draft No. 2, 2002).
In contending that a defendant should be liable for the entire amount of damage unless it meets its burden of proof as to apportionment, Morlock relies on the New-bury line of cases from other jurisdictions, citing policies having their origin in the single indivisible injury rule.5 It seems to me, however, that the pre-existing condition situation warrants different treatment than that involving multiple tortfeasors. Generally, the burden of proof is placed on the party with the greatest access to the relevant evidence. With multiple tortfea-sors, “they presumably can furnish evidence as to their degree of contribution to the plaintiffs injury. But, if the plaintiff is suffering from a preexisting injury, he or she is in the best, position to furnish information on the degree to which that injury was aggravated by the defendant’s act.” LaMoureaux v. Totem Ocean Trailer Express, Inc., 632 P.2d 539, 545 (Alaska 1981).
*166When a pre-existing condition is a “competitor for causal significance,” juries are instructed “to do the best they can to make the distinction” between the preexisting condition and the “ ‘aggravation’ brought about by the defendant’s negligent conduct.” David W. Robertson, The Common Seme of Cause in Fact, 75 Texas L.Rev. 1765, 1795 (1997) (citing LaMoureaux, 632 P.2d at 544; Epps v. City of Baton Rouge, 604 So.2d 1336, 1345 (La.Ct.App.1992)). “The plaintiff is ‘relieve[d] of proving with great exactitude the amount of aggravation,’ but ultimately should recover only for the aggravation and not for the effects of the preexisting condition.” Robertson, supra, at 1795 (quoting LaMoureaux, 632 P.2d at 544).
The issue of prior accidents, injuries, and conditions is not, as the Morlocks suggest, an affirmative defense for which the defendant bears the burden of proof; it is simply a defense to allegations of liability. Generally, an affirmative defense is new matter outside the complaint which constitutes a defense; or new matter which, assuming the complaint to be true, is a defense to it. 2 Douglas D. McFarland & William J. Kep'pel, Minnesota Civil Practice § 1151 (3d ed.1999). Morlock asserted that the accident caused his injuries. St. Paul Guardian disputed the extent of its liability for Morlock’s injuries. St. Paul Guardian’s general denial of liability did not constitute an affirmative defense. Under well-established law, it thus remained Morlock’s burden to prove that the accident caused his claimed damages. The pre-existing condition instruction given to the jury, however, alleviated Morlock of this burden. As the court of appeals concluded, the instruction given in this case misstated Minnesota law and improperly extended principles underlying joint and several liability. Morlock v. St. Paul Guardian Ins. Co., 632 N.W.2d 268, 271 (Minn.App.2001) (citing Blatz v. Allina Health Sys., 622 N.W.2d 376, 390 (Minn.App.), rev. denied (Minn. May 16, 2001)).
The Morlocks assert that even if the instruction was erroneous, St. Paul Guardian is nevertheless not entitled to a new trial given the manner of litigation. St. Paul Guardian conceded liability for the herniated discs that required surgery but disputed the remaining amount of damages caused by the accident. At trial, relevant evidence included Morlock’s medical records documenting Morlock’s history of low back trouble dating back to 1957 as well as the opinion of St. Paul Guardian’s expert that Morlock’s post-automobile accident back complaints were due to pre-existing conditions and the opinions of Morlock’s physicians to the effect that Morlock’s preexisting conditions had little to do with his post-accident problems.
This was not, as the majority posits, an “all-or-nothing scenario as to damage claims.” In arguments related to the propriety of certain jury instructions, counsel for the Morlocks acknowledged that evidence adduced at trial could permit the inference that Morlock’s preexisting condition contributed to the harm for which damages were sought; and in his closing remarks to the jury, counsel told the jury that CIVJIG 91.40 was a “very, very important instruction,” read the instruction to the jury and then told the jury that if it could not separate damages caused by the preexisting condition from those caused by the accident, St. Paul Guardian was “responsible for all the damages. That’s the law.” As the trial court noted during post-trial motions, had the burden in apportioning harm not been shifted to St. Paul Guardian, “there may very well be a different result.” Inasmuch as there was evidence permitting the factfinder to make a causal apportionment, it was “entirely possible if not probable that the jury used the district court’s erroneous instruction to *167conclude that Guardian was liable for all of Morlock’s damages, notwithstanding that not all of those damages were due to the 1996 accident.” Morlock, 632 N.W.2d at 272.
I would affirm.
. From 1963 through 1999, CIVJIG 163 covered damages in cases of pre-existing conditions:
A person who has a defect or disability at the time of an accident is nevertheless entitled to damages for any aggravation of such preexisting condition, even though the particular results would not have followed if the injured person had not been subject to such pre-existing condition. Damages are limited, however, to those results which are over and above those which normally followed from the preexisting condition, had there been no accident.
4 Minnesota District Judges Association, Minnesota Practice, Jury Instruction Guides— Civil, JIG 163 (3d ed.1986).
. The current pattern jury instruction, CIVJIG 91.40, provides:
There is evidence that (plaintiff) had a pre-existing disability or medical condition at the time of the accident.
(Defendant) is liable only for any damages that you find to be directly caused by the accident.
If you cannot separate damages caused by the pre-existing disability or medical condition from those caused by the accident, then (defendant) is liable for all of the damages.
4A Minnesota District Judges Association, Minnesota Practice, Jury Instruction Guides— Civil, JIG 91.40 (4th ed.1999).
. Canada arose from successive lead-paint poisoning suffered by the minor plaintiff at separate rental properties. The trial court determined that plaintiff's injuries were divisible, a ruling not challenged on appeal, and went on to instruct the jury to apportion among multiple tortfeasors damages between pre-July 1992 lead poisoning and damages occurring after that time. Canada by Landy v. McCarthy, 567 N.W.2d 496, 508 n. 7 (Minn.1997).
. See Mark M. Hager, What’s (Not!) in a Restatement? ALI Issue-Dodging on Liability Apportionment, 33 Conn. L.Rev. 77, 94 (2000); Frank J. Vandall, A Critique of the Restatement (Third), Apportionment as it Affects Joint and Several Liability, 49 Emory Law J. 565, 614-15 (2000).
. See, e.g., Newbury v. Vogel, 151 Colo. 520, 379 P.2d 811, 813 (1963) ("plaintiff was entitled to an instruction advising the jury that if they could not apportion the disability between the preexisting arthritis and the trauma then the defendant was liable for the entire damage resulting from the disability"); Lovely v. Allstate Ins. Co., 658 A.2d 1091, 1092 (Me.1995) (applying Newbury single injury rule); Tingey v. Christensen, 987 P.2d 588, 592 (Utah 1999) (recognizing Newbury rule). It should be remembered that the Newbury rule predated the comparative negligence scheme which was aimed at dealing with the unfairness of the all-or-nothing rule of contributory negligence. Professor Boston believes that the "same all-or-nothing unfairness that compelled the abandonment of the bar of contributory negligence should equally compel the abandonment of a rule of indivisible harm or indivisible causation." Gerald W. Boston, Apportionment of Harm in Tort Law: A Proposed Restatement, 21 U. Dayton L.Rev. 267, 372 (1996).