JUSTICE RATHJE delivered the opinion of the court:

This appeal presents the question of whether a defendant mustpresent medical or other competent evidence of a causal orrelevancy connection between a plaintiff's prior injury, prioraccident, or preexisting condition and the injury at issue.

BACKGROUND

This action arose from an automobile accident betweenplaintiff, Mark Voykin, and decedent, Gordon DeBoer. Theevidence at trial established that, on January 31, 1996, decedent'scar collided with the rear of plaintiff's car. Although plaintiff didnot seek medical attention on the evening of the accident, the nextmorning, at his wife's suggestion, he sought treatment from Dr.Chinyung See for neck and back pain. Subsequently, in the circuitcourt of Lake County, plaintiff sued decedent for injuries sufferedin the accident. After suit had been filed, but before trial, decedentdied, and his estate was substituted as defendant.

During the trial, plaintiff sought to bar and defendant soughtto introduce evidence that, in April 1991, approximately five yearsbefore the accident, plaintiff had suffered an injury to his lowerback. Plaintiff argued that such evidence was not admissible unlessdefendant presented expert testimony demonstrating that the priorand present injuries were causally related. Defendant contendedthat expert testimony was not necessary because the injuries wereto the same part of the body and continuity existed between theinjuries. The trial court allowed defendant to question plaintiff andDr. See about this prior injury and to introduce evidence relatingto the treatment of plaintiff's prior injuries. The trial court alsoallowed defendant to introduce evidence that plaintiff hadpreviously suffered "neck problems" and had been treated forcarpel tunnel syndrome.

After defendant rested, the trial court granted plaintiff'smotion for a directed verdict as to defendant's negligence. Thecourt specifically stated, however, that it was not ruling on thequestions of causation or injury. The jury returned a verdict indefendant's favor, and the trial court entered judgment on theverdict. In his post-trial motion, plaintiff argued that he shouldreceive a new trial because defendant should not have beenpermitted to introduce evidence of plaintiff's prior injury withoutproviding expert testimony to demonstrate a causal connectionbetween the past and present injuries. The trial court deniedplaintiff's motion, and plaintiff timely appealed.

The appellate court reversed and remanded for a new trial.The court examined recent decisions from other districts of theappellate court and concluded that evidence of prior injuries shouldnot be admitted unless the defendant presents "evidence ofcausation between prior and present injuries." 306 Ill. App. 3d 689,695. The court further explained that expert testimony wouldnormally be necessary if the injuries were "complex." 306 Ill. App.3d at 696. Thereafter, this court granted defendant's petition forleave to appeal (177 Ill. 2d R. 315). We also granted the motionsof the Illinois Association of Defense Trial Counsel and the IllinoisTrial Lawyers Association to file briefs as amici curiae.

ANALYSIS

History of the Same Part of the Body Rule

The "same part of the body rule," a doctrine developed in theappellate court, permits a defendant to introduce evidence that theplaintiff has previously suffered injuries similar to those at issue.Brown v. Baker, 284 Ill. App. 3d 401, 404 (1996). Under the samepart of the body rule, evidence of a prior injury is admissiblewithout any showing that it is causally related to the present injuryas long as both the past and present injuries affected the same partof the body. Brown, 284 Ill. App. 3d at 404. If the injury is not tothe same part of the body, the defendant must demonstrate a causalconnection between the current injury and the prior injury. Baileyv. Wilson, 299 Ill. App. 3d 297, 303 (1998); see also Elliott v.Koch, 200 Ill. App. 3d 1, 14 (1990).

Although this court has never addressed the applicability of thesame part of the body rule, the appellate court universally appliedthe rule until 1996. At that time, the Appellate Court, Fifth District,reconsidered the rule. The court noted that, under the rule as it wascurrently being applied, a childhood knee injury could arguably beadmissible in an action involving a later knee injury, "without anyfurther showing of relevance or causation, even if the prior injuryhad completely healed and been symptom free for decades."Brown, 284 Ill. App. 3d at 404-05. The court concluded that sucha rule of evidence invited the jury to speculate on the nexusbetween the prior and current injuries. Brown, 284 Ill. App. 3d at405. The court concluded that, if the prior injury has "long sincehealed and has shown no recurring symptoms, a defendant shouldnot be permitted to introduce evidence of the prior injury withoutestablishing causation." Brown, 284 Ill. App. 3d at 405.

Subsequently, the First District relied upon Brown andconcluded that, "absent competent and relevant evidence of acausal connection between the preexisting condition and the injurycomplained of, evidence of the preexisting condition isinadmissible." Cancio v. White, 297 Ill. App. 3d 422, 430 (1998);see also Lagestee v. Days Inn Management Co., 303 Ill. App. 3d935, 946-47 (1999) (holding that "the defendant is required topresent medical or other competent evidence of a causal orrelevancy connection between plaintiff's prior injury, prioraccident, or preexisting condition and the injury at issue as aprerequisite of admissibility").

After Cancio was decided, the Fourth District reconsideredand reaffirmed the same part of the body rule. Bailey, 299 Ill. App.3d at 304. In so doing, the court held that "[a]s long as there issome evidence of the nature, extent, duration, or treatment of theprevious injury, an independent showing of causation isunnecessary." Bailey, 299 Ill. App. 3d at 304.

Plaintiff asks us to follow the reasoning of Brown, Cancio, andLagestee and to discard the same part of the body rule. Defendantrequests that we adopt the same part of the body rule.

Shifting the Burden of Proof

Defendant's chief argument in favor of the same part of thebody rule is that requiring a defendant to present evidence ofcausation impermissibly shifts the burden of proof to defendant.The problem with defendant's argument is that this court rejectedit in 1962.

In Caley v. Manicke, 24 Ill. 2d 390 (1962), the plaintiffrecovered $20,000 for injuries sustained in a car accident. Onappeal, the defendant argued that the trial court erred in strikingevidence of the plaintiff's prior and subsequent accidents. Thiscourt, in rejecting that claim, noted that the issue "was fully andcorrectly resolved by the Appellate Court and we adopt its viewsin such regard." Caley, 24 Ill. 2d at 395.

In Caley, the defendant sought to question the plaintiffregarding accidents occurring before and after the accident at issue.The defendant's theory was that these other accidents were thecause of the plaintiff's injuries. The defendant contended that,because the plaintiff always bears the burden of proving proximatecause, the defendant did not need to demonstrate a connectionbetween the other accidents and the plaintiff's injuries.

The appellate court rejected the defendant's argument. Itexplained that, although the burden of proof never shifts betweenthe parties, the "burden of going forward with the evidence mayshift from party to party." Caley v. Manicke, 29 Ill. App. 2d 323,327 (1961). The court further reasoned that, although thedefendant bore the burden of demonstrating the connectionbetween the other accidents and the plaintiff's injuries, that did notmean that the burden of proof shifted to the defendant. Caley, 29Ill. App. 2d at 327. The court explained:

"Proximate cause was a part of plaintiff's case. It was asindispensable as the elements of defendant's negligence,plaintiff's freedom therefrom, and damage. Without it,plaintiff's action would have failed. But when he has bornethe burden of proof and established the material elementsnecessary to make out a case, it is then the defendant'sright, but certainly not his duty-to put on his defense. Thisis not shifting the burden of proof. One cannot be said tohave a burden if one may pick it up or not as he pleases.Obviously if there is evidence negative of causation, adefendant should show it, but the law in according him theprivilege of going forward in no wise shifts to him theburden of proof as the law knows that phrase." (Emphasisadded.) Caley, 29 Ill. App. 2d at 328-29.

The court then explained that, although other causes of theinjury may be a relevant area of inquiry, "the evidence elicited onthis cross-examination does not establish even remotely, a possible'cause' or 'causes' of plaintiff's injuries." Caley, 29 Ill. App. 2d at329-30. The court emphasized that whether evidence of the otheraccidents was admissible was nothing more than a question ofrelevancy:

"To elevate testimony that sometime before the rear-endaccident in question plaintiff had been bumped, bending atrunk handle, and that after the accident and before theonset of a more serious lower-back complaint, his fronthead-light was struck, to the status of contributing orintervening causes, suitable for submission to triers of thefact, is to misread standard of relevancy required as pre-conditions of admissibility. That this requirement devolveson the defendant to the same extent as on plaintiff initiallyin presenting his case, which it does, does not shift theburden of proof, or indeed have anything to do with it. Itis a question of relevancy, pure and simple." (Emphasisadded.) Caley, 29 Ill. App. 2d at 330.

We now reaffirm the analysis we adopted in Caley. Requiringa defendant to demonstrate a causal relationship between a priorand present injury in no way shifts the ultimate burden of proof.Instead, it simply requires a defendant demonstrate that theevidence he wishes to present is relevant to the question at issue,viz., whether the defendant's negligence cause the plaintiff's injury.

Relevancy

A review of the appellate court opinions reveals that the samepart of the body rule is nothing more than a bright-line relevancystandard. Essentially, the same part of the body rule provides that,if a plaintiff has previously suffered an injury to the same part ofthe body, then that previous injury is automatically relevant to thepresent injury simply because it affected the same part of the body.With this conclusion, we cannot agree.

Relevant evidence is evidence that has "any tendency to makethe existence of any fact that is of consequence to thedetermination of the action more probable or less probable than itwould be without the evidence." Fed. R. Evid. 401; see People v.Monroe, 66 Ill. 2d 317, 322 (1977) (adopting Rule 401); see alsoMarut v. Costello, 34 Ill. 2d 125, 128 (1966) (holding thatevidence is relevant if it "tends to prove a fact in controversy orrenders a matter in issue more or less probable"). Relevancy is"tested in the light of logic, experience and accepted assumption asto human behavior." Marut, 34 Ill. 2d at 128. However," '[r]elevancy is not an inherent characteristic of any item ofevidence but exists only as a relation between an item of evidenceand a matter properly provable in the case.' " Monroe, 66 Ill. 2d at322, quoting Fed. R. Evid. 401, Advisory Committee's Note.

For evidence of a prior injury to be admissible at trial, thatevidence must be relevant. The evidence of the prior injury mustmake the existence of a fact that is of consequence either more orless probable. In cases such as the one before this court, thedefendant seeks to introduce evidence of the prior injury for one ofthree purposes: (1) to negate causation; (2) to negate or reducedamages; or (3) as impeachment.

With respect to causation, evidence of a previous injury isrelevant only if it tends to negate causation or injuries. It is wellsettled that a defendant need not be the only cause to be held liablefor an injury; rather, it is sufficient that the defendant is a cause.Nelson v. Union Wire Rope Corp., 31 Ill. 2d 69, 88 (1964).Moreover, a defendant is not relieved of liability simply because theonly injury suffered by a plaintiff is an aggravation of a previousinjury. Balestri v. Terminal Freight Cooperative Ass'n, 76 Ill. 2d451, 455 (1979). Thus, for a prior injury to be relevant tocausation, the injury must make it less likely that the defendant'sactions caused any of the plaintiff's injuries or an identifiableportion thereof.

Even if the prior injury does not negate causation, it may stillbe relevant to the question of damages. For example, the priorinjury may be relevant to establish that the plaintiff had apreexisting condition for which the defendant is not liable and thatthe defendant is liable only for the portion of the damages thataggravated or increased the plaintiff's injury.

Additionally, a prior injury may be relevant as impeachment.For example, a plaintiff may be examined with respect to his failureto disclose to his physician that he has previously suffered an injuryto the same part of the body. Similarly, an expert may be examinedabout whether his opinion would change if the expert was awareof the plaintiff's prior injury. This does not mean, however, thatevery undisclosed prior injury to the same part of the body isgrounds for impeachment. Just as with the substantive admissionof evidence, trial courts should not permit inquiry into this areaunless the prior injury is relevant to a fact in consequence, i.e.,whether the prior injury negates causation or negates or reducesthe defendant's damages.

Necessity of Expert Testimony

The question remains, however, whether expert testimony isnecessary to determine whether the prior injury is relevant to thecurrent injury. In a similar context, namely, medical malpracticecases, this court has recognized that expert testimony is normallynecessary "because jurors are not skilled in the practice of medicineand would find it difficult without the help of medical evidence todetermine any lack of necessary scientific skill on the part of thephysician." Walski v. Tiesenga, 72 Ill. 2d 249, 256 (1978).Nevertheless, expert testimony is not required in medicalmalpractice actions if "the physician's conduct is so grosslynegligent or the treatment so common that a layman could readilyappraise it." Walski, 72 Ill. 2d at 256.

We believe that similar considerations should govern here.Without question, the human body is complex. A prior foot injurycould be causally related to a current back injury, yet a prior injuryto the same part of the back may not affect a current back injury.In most cases, the connection between the parts of the body andpast and current injuries is a subject that is beyond the ken of theaverage layperson. Because of this complexity, we do not believethat, in normal circumstances, a lay juror can effectively oraccurately assess the relationship between a prior injury and acurrent injury without expert assistance. Consequently, weconclude that, if a defendant wishes to introduce evidence that theplaintiff has suffered a prior injury, whether to the "same part ofthe body" or not, the defendant must introduce expert evidencedemonstrating why the prior injury is relevant to causation,damages, or some other issue of consequence. This rule appliesunless the trial court, in its discretion, determines that the naturesof the prior and current injuries are such that a lay person canreadily appraise the relationship, if any, between those injurieswithout expert assistance.

We now to apply these principles to the action before us. Todo so, we need not examine every piece of evidence introduced bydefendant. Our review reveals that the trial court erred in allowingdefendant to introduce evidence of plaintiff's prior "neckproblems" and that this alone denied plaintiff a fair trial.

At trial, both plaintiff and Dr. See testified that the day afterthe accident plaintiff complained of pain in his back and his neck.During defendant's case in chief, defendant introduced medicalrecords revealing that, when plaintiff was treated for carpal tunnelsyndrome, he told his physical therapist that he had "neckproblems" that were "secondary to playing hockey since he was 6."Defendant elicited this same information from plaintiff on cross-examination. During closing argument, defendant argued that thejury should find in favor of defendant in part because of plaintiff'sprevious complaints of neck pain.

Even a cursory review of this evidence reveals that it shouldhave been excluded. This evidence does not come close todemonstrating what plaintiff's "neck problems" were, when hesuffered them, or when he last suffered from symptoms. Nothingabout the evidence presented by defendant has any tendency tomake it less likely that defendant caused plaintiff's neck injury orthat defendant caused plaintiff to suffer damages. Without experttestimony establishing both the nature of plaintiff's prior "neckproblems" as well as the relationship between those prior problemsand plaintiff's current claim, an average juror could not readilyappraise the effect of the prior problems upon plaintiff's currentclaim. Consequently, this evidence should have been excluded.

Because the plaintiff's claim of an injury to his neck was asignificant portion of his claim and because the defense introducedno other evidence countering plaintiff's claim of a neck injury, weconclude that this erroneous introduction of evidence prejudicedplaintiff. Because this conclusion requires a new trial, we need notaddress the question of whether the trial court erred in allowing theintroduction of evidence of plaintiff's prior back injury or histreatment for carpal tunnel syndrome. Instead, we believe that thetrial court on remand is in the best position to address theadmissibility of this evidence under the new standard enunciatedhere.

CONCLUSION

Because the trial court erroneously allowed the admission ofplaintiff's "neck problems," we affirm the judgment of the appellatecourt, which reversed the judgment of the trial court and remandedthis cause for a new trial.

Appellate court judgment affirmed.

JUSTICE HEIPLE, dissenting:

In this lawsuit, plaintiff alleges that he suffered neck and backpain which resulted from a car accident with defendant. At trial,defendant introduced evidence that plaintiff had received treatmentfor neck and back pain prior to his current accident. The juryawarded plaintiff no damages. The majority holds that the trialcourt erred in admitting evidence of plaintiff's prior neck and backpain without foundation testimony first being laid to link the earlierinjuries to those plaintiff allegedly suffered in the current accident.

By today's decision, the majority rejects the long-establishedrule that, without presenting expert medical foundation evidence,a defendant may introduce evidence of plaintiff's prior injury solong as that injury is to the same part of plaintiff's body. See Baileyv. Wilson, 299 Ill. App. 3d 297, 304 (1998); Elliott v. Koch, 200Ill. App. 3d 1, 15-16 (1990); Elberts v. Nussbaum Trucking, Inc.,97 Ill. App. 3d 381, 384 (1981); Palsir v. McCorkle, 70 Ill. App.2d 425, 431 (1966). This is an evidentiary rule of reason andconvenience. The plaintiff, of course, is free to present evidence torebut any linkage to the earlier injuries.

The majority bases its decision on its fear that allowingevidence of a plaintiff's prior injuries without expert foundationtestimony "invite[s] the jury to speculate on the nexus between theprior and current injuries." Slip op. at 3. The majority worries thatretaining the same part of the body rule would permit defendantsto introduce evidence of a plaintiff's prior injury " 'without anyfurther showing of relevance or causation, even if the prior injuryhad completely healed and been symptom free for decades.' " Slipop. at 3, quoting Brown v. Baker, 284 Ill. App. 3d 401, 404-05(1996). These concerns are unfounded.

If a prior injury has "completely healed" and the plaintiff has"been symptom free for decades," the plaintiff has the opportunityof rebutting defendant's evidence with competent medicaltestimony, most likely plaintiff's own treating physician. Unlike themajority, I am confident that jurors are quite capable ofunderstanding a plaintiff's argument that his prior injury has "longsince healed" (see slip op. at 3) and is therefore distinguishablefrom the injury for which he presently seeks money damages. If, asthe majority also worries, there are medical complexities regardingthe relationship between the prior and current injuries, thesequestions are resolvable through the evidentiary and trial processeswhich are available to the plaintiff and his doctor. What is at issuehere is an evidentiary question of weight, not admissibility. On thatissue, the majority opinion misses the point. The trial court'sadmission of defendant's evidence comports with both commonsense and the long-standing rule to that effect.

Accordingly, I respectfully dissent.

JUSTICE MILLER joins in this dissent.