OPINION

HURWITZ, Justice.

¶ 1 The issue before us is whether A.R.S. § 12-2604(A) (Supp. 2008), which governs proof of the standard of care in medical malpractice cases, violates the separation of powers doctrine.

I.

¶ 2 Scott Seibel, M.D., an anesthesiologist, administered a spinal epidural to Laura Seisinger in 2002. Two years later, Seisinger filed a complaint against Siebel alleging malpractice. Seisinger subsequently disclosed that J. Antonio Aldrete, M.D., would testify as an expert about the appropriate standard of care.

¶ 3 When the defendant in a medical malpractice action is a specialist, § 12-2604(A) requires an expert witness on the standard of care to have devoted a majority of his professional time in the year preceding the incident at issue to active clinical practice or teaching in the same specialty. Dr. Siebel filed a motion contending that Dr. Aldrete, a retired anesthesiologist, did not meet the temporal practice or teaching requirement in § 12-2604(A)(2). Seisinger did not dispute the motion's factual assertions, but argued that § 12-2604(A) conflicts with Arizona Rule of Evidence 702 and violates the separation of powers doctrine set forth in Article 3 of the Arizona Constitution. Rule 702 provides that "a witness qualified as an expert by knowledge, skill, experience, training, or education" may provide testimony that "will assist the trier of fact to understand the evidence or to determine a fact in issue."

¶ 4 The superior court rejected Seisinger's constitutional argument and granted the motion in limine. The court gave Seisinger additional time to disclose a new expert qualified under § 12-2604(A). After she failed to do so, the court granted Dr. Siebel's motion to dismiss.

¶ 5 The court of appeals reversed. Seisinger v. Siebel, 219 Ariz. 163, 164-65 ¶ 1, 195 P.3d 200, 201-02 (App. 2008). It held that § 12-2604(A) conflicts with Rule 702 because the statute categorically excludes potential experts qualified to testify under the Rule. Id. at 166-67 ¶¶ 9-13, 195 P.3d at 203-04. The court of appeals also held that § 12-2604(A) did not establish substantive rights. Id. at 169-70 ¶¶ 20-21, 195 P.3d at 206-07. The court therefore found that the statute violated the separation of powers doctrine by infringing upon this Court's authority under Article 6, Section 5( 5) of the Arizona Constitution to promulgate procedural rules. Id. at 170 ¶ 22, 195 P.3d at 207.

¶ 6 We granted review because the case involves the constitutionality of a statute and the issues presented are of statewide importance. See ARCAP 23(c). We have jurisdiction under Article 6, Section 5( 3) of the Arizona Constitution and A.R.S. § 12-120.24 (2003).

II. A.

¶ 7 The Arizona Constitution commands that the legislative, executive, and judicial departments "shall be separate and distinct, and no one of such departments shall exercise the powers properly belonging to either of the others." Ariz. Const. art. 3. The Constitution also vests this Court with "[p]ower to make rules relative to all procedural matters in any court." Id. art. 6, § 5(5). Rules of evidence "have generally been regarded as procedural in nature." State ex rel. Collins v. Seidel, 142 Ariz. 587, 590, 691 P.2d 678, 681 (1984).

¶ 8 Although we have occasionally said that procedural rulemaking power is vested "exclusively" in this Court, State v. Hansen, 215 Ariz. 287, 289 ¶ 9, 160 P.3d 166, 168 (2007); Daou v. Harris, 139 Ariz. 353, 357-58, 678 P.2d 934, 938-39 (1984), this statement is in some respects an oversimplification. A statutory procedural enactment is not automatically invalid. See Seidel, 142 Ariz. at 591, 691 P.2d at 682 ("That we possess the rule-making power does not imply that we will never recognize a statutory rule."). Rather, we recognize "reasonable and workable" statutory enactments that supplement rather than conflict with rules we have promulgated. Id. (citation and internal quotation marks omitted); see Readenour v. Marion Power Shovel, 149 Ariz. 442, 445, 719 P.2d 1058, 1061 (1986) (defining determinative issue as whether the statute "supplement[s] rather than contradict[s]" an evidentiary rule). Therefore, it is more accurate to say that the legislature and this Court both have rulemaking power, but that in the event of irreconcilable conflict between a procedural statute and a rule, the rule prevails.

¶ 9 The legislature thus cannot repeal a rule of procedure or evidence. Seidel, 142 Ariz. at 591, 691 P.2d at 682. But a statute may "contradict" or effectively abrogate a rule even if there is no express repeal. Accordingly, the legislature cannot enact a statute that "provides an analytical framework contrary to the rules" of evidence. Barsema v. Susong, 156 Ariz. 309, 314, 751 P.2d 969, 974 (1988).

¶ 10 Determining whether a statute unduly infringes on our rulemaking power requires analysis of the particular rule and statute said to be in conflict. Our cases provide some guidance on purported conflicts between statutes and rules of evidence. In Readenour this Court upheld against a separation of powers attack A.R.S. § 12-686(2), which makes inadmissible "as direct evidence of a defect" evidence of changes made by the manufacturer after "the product was first sold by the defendant." 149 Ariz. at 444 n. 1, 719 P.2d at 1060 n. 1. The allegedly conflicting evidence rule was Rule 407, which makes remedial measures taken after an event supposedly giving rise to liability inadmissible "to prove negligence or culpable conduct." Rule 407 excludes from its general prohibition evidence offered for other purposes, such as to prove "ownership, control, or feasibility of precautionary measures." Readenour argued that the statute, by making inadmissible post- sale changes, conflicted with the Rule, which excludes only post- incident changes. Readenour 149 Ariz. at 445, 719 P.2d at 1061.

¶ 11 We began from the proposition that "it is our duty to save a statute, if possible, by construing it so that it does not violate the constitution." Id. (citing Ariz. Downs v. Ariz. Horsemen's Found., 130 Ariz. 550, 554, 637 P.2d 1053, 1057 (1981)). We therefore interpreted § 12-686(2), which prohibits evidence of changes only "as direct evidence of a defect," as permitting the uses of such evidence allowed under Rule 407. Id. This interpretation avoided any conflict between the statute and the exceptions in the Rule.

¶ 12 We then concluded that the statute's application to pre-injury but post-sale changes did not conflict with the Rule. Id. The Rule is silent on the admissibility of post-sale, pre-injury changes, so the statute did not expressly abrogate the Rule. Nor did the statute undermine the purposes of Rule 407. We concluded that the policy of the Rule is to encourage remedial measures, the probative value of the evidence excluded is not high, and the extension of the prohibition to this period fosters the policy embodied in the Rule. Id., at 445-46, 719 P.2d at 1061-62. We therefore "defer[red] to the legislative decisions regarding the use or exclusion of relevant evidence to promote substantive goals of public policy such as accident prevention." Id. at 446, 719 P.2d at 1062. We also noted that § 12-686(2) "is similar to a privilege statute, having both procedural and substantive aspects." Id.

¶ 13 In contrast, in Barsema we found that a statute unconstitutionally conflicted with a rule of evidence. There, the statute at issue, A.R.S. § 12-569, prohibited "for any purpose" the admission of evidence that a witness has been or is covered by a certain type of medical malpractice insurer or has a financial interest in the operation of such an insurer. Barsema, 156 Ariz. at 311-12, 751 P.2d at 971-72. The conflicting rule was Evidence Rule 411, which prohibits admission of evidence that "a person was or was not insured against liability . . . upon the issue whether the person acted negligently or otherwise wrongfully," but allows admission of such evidence when offered for another purpose, such as proof of "bias or prejudice of a witness."

¶ 14 Under Rule 411, evidence that a witness was insured could be admitted for purposes other than establishing liability if the trial judge found that it met the general Rule 402 requirement of relevancy and the Rule 403 requirement that its probative value was not "substantially outweighed by the danger of unfair prejudice" or other factors. Barsema, 156 Ariz. at 313, 751 P.2d at 973. In contrast, § 12-569 prohibited the use of such evidence "for any purpose." We were thus unable, as in Readenour, to construe the statute to avoid conflict with the rule. Instead, the mandate of Rule 411 — that evidence of insurance offered for purposes other than to establish liability is to be evaluated on a case-by-case basis under Rules 402 and 403 — had been superseded by a statutory regime in which the evidence was always excluded. We therefore concluded that § 12-569 unduly infringed on our rulemaking power because it does not "merely supplement[]" Rule 411, but rather "provides an analytical framework contrary to the rules." Id. at 314, 751 P.2d at 974.

B.

¶ 15 In this case, the court of appeals held that § 12-2604(A) conflicted with Rule 702 "because the statute cannot be harmonized with the Rule." Seisinger, 219 Ariz. at 169 ¶ 19, 195 P.3d at 206. We agree.

¶ 16 Rule 702 permits expert testimony to "assist the trier of fact to understand the evidence or to determine a fact in issue" when a witness is "qualified as an expert by knowledge, skill, experience, training, or education." "The test of whether a person is an expert is whether a jury can receive help on a particular subject from the witness. The degree of qualification goes to the weight given the testimony, not its admissibility." State v. Davolt, 207 Ariz. 191, 210 ¶ 70, 84 P.3d 456, 475 (2004) (citation omitted).

¶ 17 Dr. Siebel argues that there is no conflict between the statute and the Rule because § 12-2604(A) simply assures that an expert witness has the qualifications required by the Rule. But the statute makes clear that its requirements are in addition to those in Rule 702. Section 12-2604(C) provides that "[t]his section does not limit the power of the trial court to disqualify an expert witness on grounds other than the qualifications set forth in this section." The expert therefore must both possess the qualifications required by Rule 702 and, satisfy the additional requirements of § 12-2604(A).

¶ 18 Thus, as the court of appeals noted, § 12-2604(A) "precludes a witness who is otherwise qualified under Rule 702 from testifying in a medical malpractice case unless he or she meets the additional criteria set forth in the statute." Seisinger, 219 Ariz. at 167 ¶ 13, 195 P.3d at 204. To be sure, a trial judge could conclude under Rule 702 that a particular physician who has not recently practiced or taught is not qualified to testify about the current standard of care. But, particularly when the standard of care has not materially changed during the period after a physician left active practice or teaching, a trial judge might also well conclude that the witness remains qualified through "knowledge, skill, experience, training, or education" to assist the jury through expert testimony. As to such a witness, the statute automatically produces a different result than the Rule might produce. Indeed, such was the obvious intent of the statute: It is designed to limit which physicians are qualified to express expert opinions. See Minutes of Meeting: Hearing on S.B. 1036 Before the H. Comm. on Health, 47th Leg., 1st Reg. Sess. (Ariz. Mar. 23, 2005) (statement of Sen. Robert Cannell, co-sponsor of measure containing § 12-2604(A), that "physicians do not want retired physicians to testify against them").

¶ 19 Consequently, we agree with the court of appeals that § 12-2604(A) and Rule 702 are in "direct conflict." Seisinger, 219 Ariz. at 167 ¶ 13, 195 P.3d at 204. In interpreting a similar statute, the Michigan Supreme Court reached precisely this conclusion, noting that the legislature "envisioned and intended that the statute would often compel different qualification determinations than the rule when applied to a given case." McDougall v. Schanz, 461 Mich. 15, 597 N.W.2d 148, 153 (1999).

¶ 20 Several federal cases have held that state statutes similar to § 12-2604(A) must be applied in diversity cases notwithstanding Federal Rule of Evidence 702. See, e.g., Legg v. Chopra, 286 F.3d 286, 291-92 (6th Cir. 2002); see also McDowell v. Brown, 392 F.3d 1283, 1294-95 (11th Cir. 2004) (following Legg); Miville v. Abington Mem'l Hosp., 377 F.Supp.2d 488, 493 (E.D.Pa. 2005) (same). These cases, however, are not contrary to our conclusion that § 12-2604(A) conflicts with Arizona Rule of Evidence 702.

¶ 21 The federal cases interpret Federal Rule of Evidence 601, which provides that "in civil actions and proceedings, with respect to an element of a claim or defense as to which State law supplies the rule of decision, the competency of a witness shall be determined in accordance with State law." Therefore, the conclusion by the federal courts that the state statutes enacted a rule of competency compelled application of the state law. Moreover, the question of whether a federal court sitting in diversity applies state or federal law turns under Erie Railroad Co. v. Tompkins, 304 U.S. 64, 58 S.Ct. 817, 82 L.Ed. 1188 (1938), on notions of federalism, not separation of powers. Congress has no more ability through legislation than does the Supreme Court of the United States through rulemaking to prescribe the substantive rules of decision in diversity cases. See Hanna v. Plumer, 380 U.S. 460, 471-72, 85 S.Ct. 1136, 14 L.Ed.2d 8 (1965) ("[N]either Congress nor the federal courts can . . . fashion rules which are not supported by a grant of federal authority contained in Article I or some other section of the Constitution; in such areas state law must govern because there can be no other law.").

¶ 22 In contrast to the federal rule, Arizona Rule of Evidence 601 states: "Every person is competent to be a witness except as otherwise provided in these rules or by statute." The Arizona Rule is silent as to the effect of a conflict between a rule and a statute. Thus, even assuming that § 12-2604(A) is a rule of competency, see Kenneth S. Broun et al., McCormick on Evidence § 69, at 332 (6th ed. 2006) (characterizing the qualification requirements in Federal Rule of Evidence 702 as competency requirements), the question nonetheless remains under Article 3 and Article 6, Section 5(5), whether the statute impermissibly conflicts with Rule 702.

¶ 23 Our prior cases illustrate the point. For example, Rule 402 provides that "[a]ll relevant evidence is admissible, except as otherwise provided by the Constitution of the United States, by the Constitution of Arizona or by applicable statutes or rules." But in Readenour we stated that we "cannot let the legislature define what is relevant." 149 Ariz. at 446, 719 P.2d at 1062. And in Barsema, we struck down a statute that effectively modified the relevancy rules notwithstanding the "except as otherwise provided" language of Rule 402. Barsema, 156 Ariz. at 314, 751 P.2d at 974. Similarly, Rule 802 provides that hearsay is inadmissible "except as provided by applicable constitutional provisions, statutes, or rules." Nonetheless, in State v. Robinson, we struck down a legislative exception to the hearsay rules under the separation of powers doctrine. 153 Ariz. 191, 197-98, 735 P.2d 801, 807-08 (1987).

III. A.

¶ 24 As the court of appeals recognized, a determination that a statute and court rule cannot be harmonized is but the first step in a separation of powers analysis. Seisinger, 219 Ariz. at 169 ¶ 20, 195 P.3d at 206. If there is a conflict, as the court of appeals recognized, id., we must then determine whether the challenged statutory provision is substantive or procedural. See, e.g., Valerie M. v. Ariz. Dep't of Econ. Sec., 219 Ariz. 331, 336 ¶ 21, 198 P.3d 1203, 1208 (2009); Hansen, 215 Ariz. at 289 ¶ 9, 160 P.3d at 168 ("[W]hen a statute and a rule conflict, we traditionally inquire into whether the matter regulated can be characterized as substantive or procedural, the former being the legislature's prerogative and the latter the province of this Court."); State v. Fowler, 156 Ariz. 408, 410-11, 752 P.2d 497, 499-500 (App. 1987).

¶ 25 The court of appeals concluded that § 12-2604(A) is not substantive because "the legislative history of the enactment of § 12-2604(A) does not establish that the Arizona Legislature had concerns regarding the right to sue, limiting the right to sue or changing the burden of proof to prevail in medical malpractice cases." Seisinger, 219 Ariz. at 170 ¶ 21, 195 P.3d at 207. Even assuming the factual accuracy of this statement, however, the issue of whether an enactment is procedural or substantive cannot turn on the record made in legislative hearings. The question is instead one of law.

¶ 26 This legal inquiry, like the determination of whether a rule and a statute conflict, is mandated by fundamental concepts of separation of powers. Article 4, Part 1, Section 1 of the Arizona Constitution vests the legislature (and the people through ballot measure) with the "legislative authority" of the State. The legislature has plenary power to deal with any topic unless otherwise restrained by the Constitution. Giss v. Jordan, 82 Ariz. 152, 159, 309 P.2d 779, 783-84 (1957); Adams v. Bolin, 74 Ariz. 269, 283, 247 P.2d 617, 626 (1952). Thus, once we determine that a statute conflicting with a court-promulgated rule is "substantive," the statute must prevail. See Valerie M., 219 Ariz. at 336 ¶ 21, 198 P.3d at 1208; Hansen, 215 Ariz. at 289 ¶ 10, 160 P.3d at 168 ("[I]if [the conflicting statute] is substantive, it indisputably governs.").

¶ 27 Just as the primacy of the courts in promulgating procedural rules does not exclude a supplementary legislative role, our Constitution does not prohibit the judiciary from developing substantive law. An obvious example of this role is in the area of the common law. The legislature has expressly provided that the "common law . . . is adopted and shall be the rule of decision in all courts of this state." A.R.S. § 1-201 (2002). The American legal tradition relies on courts to make substantive law through the development of the common law. See Cronin v. Sheldon, 195 Ariz. 531, 537, 991 P.2d 231, 237 (1999) ("Courts do make law. The common law is and has been a product of the courts for hundreds of years. To adopt the common law is, by definition, to adopt the plenary role of the judiciary in its continuing development.").

¶ 28 But just as a procedural statute cannot prevail against a procedural rule validly promulgated under Article 6, judge-made substantive law is subordinated to contrary legislative acts validly adopted under Article 4. Section 1-201 recognizes this basic constitutional principle, adopting the common law only insofar as it is "not, repugnant to or inconsistent with the constitution of the United States or the constitution or the laws of this state." Thus, when a substantive statute conflicts with the common law, the statute prevails under a separation of powers analysis. See Pleak v. Entrada Prop. Owners' Ass'n, 207 Ariz. 418, 422 ¶ 12, 87 P.3d 831, 835 (2004).

1.

¶ 29 Although the basic constitutional principle of separation of powers is easily stated, the precise dividing line between substance and procedure "has proven elusive." In re Shane B., 198 Ariz. 85, 88 ¶ 9, 7 P.3d 94, 97 (2000). In an oft-quoted passage, this Court stated that

the substantive law is that part of the law which creates, defines and regulates rights; whereas the adjective, remedial or procedural law is that which prescribes the method of enforcing the right or obtaining redress for its invasion. It is often said the adjective law pertains to and prescribes the practice, method, procedure or legal machinery by which the substantive law is enforced or made effective.

State v. Birmingham, 96 Ariz. 109, 110, 392 P.2d 775, 776 (1964). This definition, while unexceptionable, unfortunately does not provide an answer in close cases. Statutes relating to evidence present particularly difficult problems, as such statutes, like rules of evidence, often have both substantive and procedural aspects. See Readenour, 149 Ariz. at 446, 719 P.2d at 1062 (noting that § 12-682(2) has "both procedural and substantive aspects"); cf. Legg, 286 F.3d at 290 ("[S]ome state evidentiary rules have substantive aspects, thereby defying the substance-procedure distinction."). The relevant inquiry therefore cannot end, as our concurring colleague suggests, see ¶ 52, infra, with the conclusion that there is some conflict between a validly enacted rule of evidence and a statute. The ultimate question is whether the statute enacts, at least in relevant part, law that effectively "creates, defines, and regulates rights."

¶ 30 Even if a litmus test as to what is substantive is not possible, our cases do provide guidance. "Because the legislature is empowered to set burdens of proof as a matter of substantive law, a valid statute specifying the burden of proof prevails over common law or court rules adopting a different standard." Valerie M., 219 Ariz. at 336 ¶ 21, 198 P.3d at 1208; see State v. Fletcher, 149 Ariz. 187, 191-93, 717 P.2d 866, 870-72 (1986) (holding that burden of proof of insanity is substantive and may be altered by the legislature). We have also recognized that common law privileges are substantive and generally subject to legislative amendment. See Readenour, 149 Ariz. at 446, 719 P.2d at 1062; State v. Whitaker, 112 Ariz. 537, 540, 544 P.2d 219, 222 (1975). Similarly, the legislature may modify the elements of common law causes of action, subject to constitutional constraints not at issue in this case. See Duncan v. Scottsdale Med. Imaging, Ltd., 205 Ariz. 306, 314 n. 2, 70 P.3d 435, 443 n. 2 (2003) (recognizing that legislature may regulate tort elements, subject to constraints imposed by Arizona Constitution article 18, § 6); cf. McDougall, 597 N.W.2d at 158-59 (noting legislative ability to modify elements of causes of action).

¶ 31 Because § 12-2604(A) provides that certain expert testimony cannot be received, Seisinger argues that it must be procedural. But, as Readenour teaches, a statute excluding evidence is not inevitably procedural; privilege statutes exclude highly relevant evidence but are nonetheless substantive. 149 Ariz. at 446, 719 P.2d at 1062. We thus approach our analysis of § 12-2604(A) with the goal of determining, absent labels, the true function of the statute. We do so mindful of our duty to construe legislation, if possible, so that it does not violate the constitution. Readenour, 149 Ariz. at 446, 719 P.2d at 1062.

2.

¶ 32 In medical malpractice actions, as in all negligence actions, the plaintiff must prove the existence of a duty, a breach of that duty, causation, and damages. Smethers v. Campion, 210 Ariz. 167, 170 ¶ 12, 108 P.3d 946, 949 (App. 2005). The "yardstick by which a physician's compliance with [his] duty is measured is commonly referred to as the `standard of care.'" Id. ¶ 13.

¶ 33 Under the common law, breach of duty in malpractice actions required proof that the defendant failed to exercise the "same care in the performing of his duties as was ordinarily possessed and exercised by other physicians of the same class in the community in which he practiced." Rice v. Tissaw, 57 Ariz. 230, 237-38, 112 P.2d 866, 869 (1941) (citing Butler v. Rule, 29 Ariz. 405, 242 P. 436 (1926)). Arizona courts have long held that the standard of care normally must be established by expert medical testimony. See, e.g., Riedisser v. Nelson, 111 Ariz. 542, 544, 534 P.2d 1052, 1054 (1975); Stallcup v. Coscarart, 79 Ariz. 42, 46, 282 P.2d 791, 793-94 (1955); Boyce v. Brown, 51 Ariz. 416, 421, 77 P.2d 455, 457 (1938). Thus, except when it was "a matter of common knowledge . . . that the injury would not ordinarily have occurred if due care had been exercised," Falcher v. Saint Luke's Hosp. Med. Ctr., 19 Ariz.App. 247, 250, 506 P.2d 287, 290 (1973), a plaintiff could not meet the burden of production under the common law absent expert testimony. Failure to produce the required expert testimony mandated judgment for the physician-defendant. See, e.g., Rodriguez v. Jackson, 118 Ariz. 13, 17, 574 P.2d 481, 485 (App. 1978).

¶ 34 Arizona courts have also long held that expert testimony on the standard of care can be presented only by a physician. Rice, 57 Ariz. at 238, 112 P.2d at 869. Therefore, a plaintiff producing only the testimony of a nurse or a pharmacologist on the standard of care has not satisfied the substantive burden of proof, and his claim is subject to summary disposition. See Rudy v. Meshorer, 146 Ariz. 467, 470, 706 P.2d 1234, 1237 (App. 1985) ("The testimony of a registered nurse cannot be used to establish the standard of care a doctor must meet."); Rodriguez, 118 Ariz. at 17, 574 P.2d at 485.

¶ 35 Arizona common law decisions requiring expert testimony from physicians in medical malpractice cases long predated the adoption of the Arizona Rules of Evidence in 1977. Thus, as a purely chronological matter, the requirement of a physician expert cannot be said to have resulted from our — promulgation of a procedural rule. More importantly, the common law doctrine is far stricter than Rule 702. Our decisions hold that a plaintiff cannot satisfy the burden of production absent an expert physician witness; failure to produce such a witness results in judgment for the defendant. In contrast, Rule 702 does not require that a plaintiff produce expert testimony in any case. Rather, the Rule simply permits expert testimony when the finder of fact would thereby be aided. Nor does Rule 702 purport to require that experts on the standard of care be physicians. Rather, the Rule leaves to the trial judge the determination of whether the witness possesses the required "knowledge, skill, experience, training or education."

¶ 36 Accordingly, even when the Dean of the College of Nursing at the University of Arizona testified that a registered nurse could, under many circumstances, be qualified both by training and experience to know the applicable standard of care, the court of appeals concluded that summary judgment for the defendant was required when there was no testimony from a medical doctor. Rodriguez, 118 Ariz. at 16-17, 574 P.2d at 484-85; see also id. (reaching same conclusion as to Ph.D. pharmacologist despite similar testimony from associate dean of medical school); Rudy, 146 Ariz. at 470, 706 P.2d at 1237 (affirming summary judgment when the only testimony on standard of care came from registered nurse).

¶ 37 In short, the requirement of expert physician testimony in a medical malpractice action is not simply the result of Rule 702 or some other procedural rule. Our decisions requiring expert physician testimony do not turn on a case-by-case examination of the training or qualifications of any individual expert. Rather, they teach that a plaintiff cannot satisfy the burden of proving a required element of the tort in the absence of a very specific kind of evidence. To establish the requisite standard of care, Arizona cases do not accept just any kind of expert witness, but rather demand a physician.

¶ 38 We therefore conclude that the requirement of expert testimony in a medical malpractice action is a substantive component of the common law governing this tort action. The common law requirement reflected a policy decision by the courts that the plaintiff's substantive burden of production could only be met by a particular kind of evidence. The common law requirement thus effectively established an element of the cause of action, by specifying the kind of proof necessary to meet the plaintiff's burden of production.

¶ 39 The common law elements of a medical malpractice action have now been partially codified by the legislature in A.R.S. § 12-563. Because § 12-563 defines the elements of a cause of action, it is plainly substantive. Section 12-2604(A) is substantive in much the same sense. It "regulates rights," Birmingham, 96 Ariz. at 110, 392 P.2d at 776, by modifying the common law to increase a plaintiff's burden of production with respect to a statutory element of the tort, departure from the standard of care. Before the enactment of § 12-2604(A), Seisinger could satisfy that burden by presenting the testimony of Dr. Aldrete, assuming he qualifies as an expert under Rule 702. After § 12-2604(A) became effective, the same evidence is not sufficient, as a matter of law, to avoid summary judgment. The statute thus did not merely alter court procedures, but rather changed the substantive law as to what a plaintiff must prove in medical malpractice actions. Cf. Peck v. Tegtmeyer, 834 F.Supp. 903, 909 (W.D.Va. 1992) ("In other words, under the statutory scheme, the standard of care is that which is testified to by an expert qualified under the statute."), aff'd mem., 4 F.3d 985 (4th Cir. 1993); Legg, 286 F.3d at 290 ("State witness competency rules are often intimately intertwined with a state substantive rule. This is especially true with medical malpractice statutes, because expert testimony is usually required to establish the standard of care.").

¶ 40 Our conclusion that § 12-2604(A) is substantive insofar as it requires a certain type of evidence to prove an element of the tort does not, as our concurring colleague suggests, "shift[] the established boundaries of judicial and legislative domains." See ¶ 60, infra. The doctrine of separation of powers does not constrain the legislature from modifying, or even abolishing, the elements of common law causes of action. See ¶ 30, supra. Nor do separation-of-powers principles restrict the legislature's ability to increase the plaintiff's common law burden of proof. See id. If separation of powers would not prevent the legislature from requiring that all medical malpractice plaintiffs prove claims beyond a reasonable doubt, it is difficult to see why the legislature cannot require a heightened level of proof of the standard of care.

¶ 41 Although we maintain plenary power over procedural rules, we do not believe that power precludes the legislature from addressing what it believes to be a serious substantive problem — the effects on public health of increased medical malpractice insurance rates and the reluctance of qualified physicians to practice here — by effectively increasing the plaintiff's burden of production in medical malpractice actions. Given the critical substantive function that our common law decisions have ascribed to expert testimony about the standard of care, § 12-2604(A) is properly viewed as a modification of that substantive common law, not merely as a change in procedure.

B.

¶ 42 We therefore conclude that insofar as § 12-2604(A) specifies the kind of expert testimony necessary to establish medical malpractice, it is substantive in nature and does not offend the separation of powers doctrine. Section 12-2604(A) was enacted in 2005. 2005 Ariz. Sess. Laws, ch. 188, § 1 (1st Reg. Sess.). This case was filed in 2004, and concerns conduct occurring in 2002. Therefore, we must decide whether § 12-2604(A) applies retroactively to actions filed before its effective date.

¶ 43 "No statute is retroactive unless explicitly declared therein." A.R.S. § 1-244 (2002). Section 12-2604 contains no such declaration. Arizona courts have "engrafted an exception" to this statute under which a statute may nonetheless have "retroactive effect if it is merely procedural." In re Shane B., 198 Ariz. at 87 ¶ 8, 7 P.3d at 96 (citation and internal quotations omitted). We have today concluded that § 12-2604(A) is not merely procedural. The statute therefore does not apply retroactively to Seisinger's claim.

IV.

¶ 44 For the reasons above, we hold that § 12-2604(A) does not violate the constitutional separation of powers doctrine. We therefore vacate the opinion of the court of appeals. This case does not present, and we today express no opinion, as to whether the statute contravenes any other constitutional provision. Because § 12-2604(A) does not apply retroactively, we also vacate the judgment of the superior court and remand for further proceedings consistent with this opinion.

CONCURRING: REBECCA WHITE BERCH, Vice Chief Justice, MICHAEL D. RYAN, and W. SCOTT BALES, Justices.