ALEXANDER, J.

[¶ 1] This case is before us on questions certified by the United States District Court for the District of Maine ( Hornby, J.) pursuant to 4 M.R.S. § 57 (2005) and M.R.App. P. 25. The questions of state law certified are: (1) "Does the Maine Human Rights Act definition of `physical or mental disability' found at 5 M.R.S.A. § 4553(7-A) require a showing of a substantial limitation on a major life activity as does its federal analogue, 42 U.S.C. § 12102(2)(A)?"; and (2) "Is Section 3.02(C) of the regulations adopted by the Maine Human Rights Commission, defining a `physical or mental impairment,' invalid because it requires a showing of a substantial limitation on a major life activity?" See Whitney v. Wal-Mart Stores, Inc., 370 F.Supp.2d 323, 327 (D.Me. 2005). We answer the federal court's first question in the negative, and the second question in the affirmative.

I. CASE HISTORY

[¶ 2] In its certification, the District Court has determined that the following facts are undisputed. Id. at 324-27. In 1998, Wal-Mart hired Stanley Whitney to work at a store location in Florida. In 2001, Whitney was offered and accepted a salaried management-level position in Maine as tire-lube express department manager at the Wal-Mart store in North Windham. In that position, he worked an average of six days and more than seventy hours per week.

[¶ 3] Soon after starting work in Maine, Whitney's health began to deteriorate. He was diagnosed with high blood pressure and "possibly serious" heart disease. Whitney requested and was granted a two-month leave of absence for further testing of his condition. When he returned to his position, Whitney had a note from his physician's assistant indicating that he should be allowed to work reduced work hours of no more than eight hours per day and forty hours per week with two consecutive days off. Whitney later amended his requested hours to work nine hours per day with two consecutive days off, and no more than forty-five hours per week.

[¶ 4] Whitney's supervisors informed him that his minimum work hours as the tire-lube manager in the North Windham store would be forty-eight to fifty-two hours per week. Wal-Mart also notified Whitney that if he could not work these minimum hours, other non-salaried department manager positions could be available to him. Whitney applied unsuccessfully for several other managerial positions before eventually accepting a position as a non-salaried department manager at the Scarborough Wal-Mart, where he continues to work.

[¶ 5] In January 2004, Whitney filed a complaint, later amended, in the Superior Court (Androscoggin County) alleging age and disability discrimination by Wal-Mart in violation of the Maine Human Rights Act (MHRA), 5 M.R.S.A. §§ 4551— 4634 (2002), as well as breach of his employment contract. He requested injunctive relief in the form of reinstatement to his former management position in North Windham, and damages. Wal-Mart removed the case to the United States District Court, asserting diversity of citizenship.

[¶ 6] Once in federal court, Wal-Mart moved for summary judgment. The magistrate judge ( Kravchuk, J.) issued a recommended decision that Wal-Mart's motion for summary judgment be granted as to all counts. The recommended decision was based, in part, on the conclusion that Maine's disability discrimination law does not provide a remedy unless the plaintiff can establish that the disability substantially limits a major life activity, consistent with the federal Americans with Disabilities Act, 42 U.S.C.A. §§ 12101- 12213 (West 2005), and that because Whitney failed to establish a prima facie case of such a substantial limitation, Wal-Mart was entitled to summary judgment. Following the recommended decision, the District Court ( Hornby, J.) certified the case for our interpretation of Maine law pursuant to 4 M.R.S. § 57 and M.R.App. P. 25.

II. LEGAL ANALYSIS

A. Jurisdiction

[¶ 7] We have jurisdiction to consider certified questions of State law posed by a federal court when such questions "may be determinative of the cause," and when "there are no clear controlling precedents in the decisions of the Supreme Judicial Court." 4 M.R.S. § 57; M.R.App. P. 25(a). The District Court has certified that our determination of the questions posed may be dispositive of the action pending before the District Court by entry of a summary judgment, and that no controlling precedents otherwise exist. Thus, our exercise of jurisdiction in this matter is proper.

B. The Maine Human Rights Act

[¶ 8] The MHRA was enacted in 1971 to acknowledge "the basic human right to a life with dignity" by preventing discrimination in "employment, housing or access to public accommodations" as well as in the extension of credit and in education. 5 M.R.S. § 4552 (2005). One of the claims brought by Whitney against Wal-Mart — and the only claim relevant to the questions before us — is a cause of action for disability discrimination pursuant to 5 M.R.S. § 4572 (2005).

[¶ 9] An employee plaintiff pursuing a claim for disability discrimination must establish that "first, [he] suffers from a disability; second, [he] is otherwise qualified, with or without reasonable accommodations, and is able to perform the essential functions of the job; and third, [he] was adversely treated by the employer based in whole or in part on [his] disability." Doyle v. Dep't of Human Servs., 2003 ME 61, ¶ 14, 824 A.2d 48, 54.

[¶ 10] When the MHRA was enacted in 1971, it included no definition of "disability" or the similar term "handicap." Congress adopted a definition of "disability" or "handicap" in the Rehabilitation Act of 1973 when it defined "handicapped individual" as a person who "has a physical or mental impairment which substantially limits one or more of such person's major life activities. . . ." P.L. 93-516, § 111(a), 88 Stat. 1617, 1619 (1974) (codified at 29 U.S.C.A. § 705(20)(B)(i) (West 1999)).

[¶ 11] Shortly after enactment of the Rehabilitation Act, the Maine Legislature added a definition of "physical or mental handicap" to the MHRA, P.L. 1975, ch. 358, § 2. Our Legislature chose not to include in the definition the "substantially limits . . . major life activities" restriction of its recently adopted federal counterpart. Instead the Legislature enacted 5 M.R.S.A. § 4553(7-A) largely as it reads today. As originally enacted, 5 M.R.S.A. § 4553(7-A) read as follows:

Physical or mental handicap. "Physical or mental handicap" means any disability, infirmity, malformation, disfigurement, congenital defect or mental condition caused by bodily injury, accident, disease, birth defect, environmental conditions or illness; and also includes the physical or mental condition of a person which constitutes a substantial handicap as determined by a physician or, in the case of mental handicap, by a psychiatrist or psychologist, as well as any other health or sensory impairment which requires special education, vocational rehabilitation or related services.

P.L. 1975, ch. 358, § 2 (codified at 5 M.R.S.A. § 4553(7-A) (1979)).

[¶ 12] This provision remained unchanged until 1991. See P.L. 1991, ch. 99, § 2. At that time, the Legislature made what it characterized as stylistic changes that included changing the word "handicap" to "disability." See L.D. 191, Statement of Fact (115th Legis. 1991) ("This bill does not change the substance of the Maine Human Rights Act, but changes the terminology from handicap to disability.").

[¶ 13] The 1991 amendment, shown as follows, adopted the definition of disability in section 4553(7-A) that exists today:

Physical or mental disability. "Physical or mental handicap disability" means any disability, infirmity, malformation, disfigurement, congenital defect or mental condition caused by bodily injury, accident, disease, birth defect, environmental conditions or illness;, and also includes the physical or mental condition of a person which that constitutes a substantial handicap disability as determined by a physician or, in the case of mental handicap disability by a psychiatrist or psychologist, as well as any other health or sensory impairment which that requires special education, vocational rehabilitation or related services.

P.L. 1991, ch. 99, § 2.

[¶ 14] With these nonsubstantive amendments, section 4553(7-A) now defines physical or mental disability as:

any disability, infirmity, malformation, disfigurement, congenital defect or mental condition caused by bodily injury, accident, disease, birth defect, environmental conditions or illness, and includes the physical or mental condition of a person that constitutes a substantial disability as determined by a physician or, in the case of mental disability, by a psychiatrist or psychologist, as well as any other health or sensory impairment that requires special education, vocational rehabilitation or related services.

5 M.R.S. § 4553(7-A) (2005).

[¶ 15] When the Legislature acted in 1991, the less-inclusive Rehabilitation Act definition had been in effect for more than seventeen years. In 1990, the year before section 4553(7-A) was amended, Congress adopted the Americans with Disabilities Act with a definition of disability similar to that in the Rehabilitation Act, "a physical or mental impairment that substantially limits one or more of the major life activities of such individual." Pub.L. 101-336, § 3(2)(A), 104 Stat. 328, 330 (1990) (codified at 42 U.S.C.A. § 12102(2)(A) (West 2005)). Legislative enactment of section 4553(7-A) without the "substantially limits . . . major life activities" language, a year after Congress originally adopted the Rehabilitation Act, and legislative amendment of section 4553(7-A) without the restrictive language, a year after such language was included in the enactment of the ADA are strongly probative of legislative rejection, not adoption, of the restrictive language.

[¶ 16] Prior to the 1991 legislation, the Maine Human Rights Commission, in 1985, had adopted some regulations to guide its enforcement activities. Among the regulations adopted by the Commission was Rule 3.02, that provides further definitions to accompany the MHRA. Included among these definitions is one for "physical or mental disability" that states: "An applicant or employee who has a `physical or mental disability' means any person who has a physical or mental impairment which substantially limits one or more of such person's major life activities, has a record of such impairment, or is regarded as having such impairment." 11 C.M.R. 94, 348, 003-2 § 3.02(C)(1) (1999).

C. Applicability of the "Substantially Limits" Language

[¶ 17] Wal-Mart and the Maine Chamber of Commerce argue that the Legislature's 1991 amendment signified, sub silentio, that the 1985 Maine Human Rights Commission regulation was a proper construction of the definition of disability that the Legislature had enacted in 1975. Nothing in the legislative history, or anywhere else, supports that view. If the stated legislative intent of the 1991 amendment was to continue and "not change the substance of the Maine Human Rights Act," then this amendment was surely neither an endorsement nor an acceptance of the restrictive language included in the Maine Human Rights Commission regulation.

[¶ 18] The reason that the Human Rights Commission's 1985 regulation cannot be viewed as receiving approval by the Legislature is quickly apparent. A year after the regulation was adopted, we held in Rozanski v. A-P-A Transport, Inc., 512 A.2d 335, 340 (Me. 1986) that a definition of disability, without the "substantially limits" language, should govern interpretation of the term "disability" or "handicap" under the MHRA.

[¶ 19] In Rozanski, plaintiffs were two truck drivers who were discharged because they failed A-P-A Transport's pre-employment x-ray screening. Although neither plaintiff had ever experienced back problems, x-rays revealed that one had a small osteophyte or spur on his spine and the other had spondylolysis. Id. at 338. The Maine Human Rights Commission determined that these latent back conditions were a physical handicap pursuant to section 4553(7-A). Id. After trial, the Superior Court determined that A-P-A Transport had discharged the plaintiffs because of a mistaken belief that their latent back conditions created a greater likelihood of disability if they engaged in the heavy work of truck driving. Id.

[¶ 20] A-P-A Transport argued that these latent back conditions were not a protected "physical or mental handicap" under the MHRA. We rejected this argument:

The [Maine Human Rights] Act defines "physical or mental handicap" as "any disability, infirmity, malformation, disfigurement, congenital defect or mental condition caused by bodily injury, accident, disease, birth defect, environmental conditions or illness. . . ." 5 M.R.S.A. § 4553(7-A) (1979). Both [plaintiffs] fit within the express terms of that definition since the asymptomatic condition of each of the men constitutes a "malformation" of the spine. Their conditions are indistinguishable from that of the railway employee involved in Maine Human Rights Commission v. Canadian Pacific Ltd., 458 A.2d 1225 (Me. 1983), who because of his asymptomatic heart murmur was deemed to be handicapped within the meaning of the Act. Similarly, [plaintiffs'] latent back conditions, which were the sole ground for their termination, are physical handicaps that entitle them to the protection of the Maine Human Rights Act.

Id. at 340.

[¶ 21] The Rozanski opinion holding that "latent" conditions are protected under the section 4553(7-A) definition, necessarily rejects the "substantially limits" qualification to the definition of disability. It was this interpretation of section 4553(7-A), by both the Maine Human Rights Commission and this Court, that governed in 1991 when the Legislature amended section 4553(7-A), without substantive change, and it is this interpretation of the law that governs today. Not surprisingly, the Chief Judge of the United States District Court has interpreted section 4553(7-A) in just this way, as having "a broader definition of `physical or mental disability'" than the Rehabilitation Act. Norton v. Lakeside Family Practice, P.A., 382 F.Supp.2d 202, 205 n. 2 (D.Me. 2005).

[¶ 22] Wal-Mart asserts that, despite this history of section 4553(7-A), we should defer to the Commission's interpretation of a statute it administers. Our standards of review require that we afford significant deference to an agency's interpretation of a statute it administers: "When the dispute involves an agency's interpretation of a statute administered by it, the agency's interpretation, although not conclusive, is entitled to great deference. . . ." Maritime Energy v. Fund Ins. Review Bd., 2001 ME 45, ¶ 7, 767 A.2d 812, 814. However, we must directly interpret an unambiguous statute according to its plain meaning. City of Bangor v. Penobscot County, 2005 ME 35, ¶ 9, 868 A.2d 177, 180. If, and only if, a statute is ambiguous do we look to extrinsic sources like agency interpretation or legislative history to assist in interpreting ambiguous terms. Competitive Energy Servs., LLC v. Pub. Utils. Comm'n, 2003 ME 12, ¶ 15, 818 A.2d 1039, 1046. An agency interpretation of a statute is invalid if it is contrary to the plain meaning of the statute. See Gulf Island Pond Oxygenation Project P'ship v. Bd. of Envtl. Prot., 644 A.2d 1055, 1059 (Me. 1994).

[¶ 23] An agency cannot, by regulation, create an ambiguity in interpretation of a statute that does not otherwise exist. Such an approach is particularly difficult when the agency's construction seeks to add words of limitation to a statute that are inconsistent with the plain meaning of the law, legislative choice in enacting the law, and subsequent judicial interpretation.

[¶ 24] Wal-Mart argues, contrary to our holding in Rozanski, that the "substantially limits" language of the federal law is incorporated into section 4553(7-A) by its reference to "substantial disability." This is a misreading of the statute. The definition of "physical or mental disability" in section 4553(7-A) includes three categories of covered conditions. Under the first category, a person is covered if he or she has "any disability, infirmity, malformation, disfigurement, congenital defect or mental condition caused by bodily injury, accident, disease, birth defect, environmental conditions or illness." Rozanski, 512 A.2d at 340 (quoting 5 M.R.S.A. § 4553(7-A) (1979)). The second category is "the physical or mental condition of a person that constitutes a substantial disability as determined by a physician or, in the case of mental disability, by a psychiatrist or psychologist. . . ." 5 M.R.S. § 4553(7-A). The third category is "any other health or sensory impairment that requires special education, vocational rehabilitation or related services." Id.

[¶ 25] Although it is possible for an individual to have a condition that meets all three categories, Rozanski establishes that meeting all of them is not a prerequisite for coverage. In Rozanski, an asymptomatic back condition was covered because it was a "malformation" of the spine, 512 A.2d at 340, regardless of the "substantial disability" language in section 4553(7-A).

[¶ 26] Section 4553(7-A) in defining disability is not ambiguous. It does not include the "substantially limits . . . a major life activity" qualification that the Legislature has chosen not to include in Maine's definition of disability.

[¶ 27] Wal-Mart and the Chamber of Commerce assert, as a matter of policy, that Maine's definition of disability, without the restriction in the federal law, may promote litigation unduly burdensome to businesses. Therefore, they urge us to, in effect, amend the MHRA to include the limitation present in federal law but not in the Maine Legislature's enacted definition of "disability." We have cautioned litigants that legislative policy arguments are more appropriately left to the executive and the Legislature to resolve. In Bertl v. Public Utilities Commission, 2005 ME 115, ¶ 11, 885 A.2d 776, 778, we rejected an appellant's argument as:

more of a policy argument than it is a legal argument, and as such, is more appropriately left for the Commission, not this Court, to consider. See, e.g., Harding v. Sheridan D. Smith, Inc., 647 A.2d 1193, 1194 (Me. 1994) (noting that "[w]hile the employer makes an interesting policy argument, such policy issues are more appropriately addressed to the Legislature").

If a legislative policy concern is valid, the appropriate body to address that concern is the Maine Legislature, it is not to seek amendment of the law by judicial action.

[¶ 28] The entire history of the adoption and judicial interpretation of the definition of disability in the MHRA leaves no ambiguity for interpretation and supports a definition of "disability" without a "substantially limits one or more of the major life activities" qualification.

D. Responses to Questions

[¶ 29] Based on the above discussion, we respond to the certified questions as follows:

[¶ 30] Question # 1 — Does the Maine Human Rights Act definition of "physical or mental disability" found at 5 M.R.S.A. § 4553(7-A) require a showing of a substantial limitation on a major life activity as does its federal analogue, 42 U.S.C. § 12102(2)(A)?

[¶ 31] We answer question # 1, "No."

[¶ 32] Question # 2 — Is Section 3.02(C) of the regulations adopted by the Maine Human Rights Commission, defining a "physical or mental impairment," invalid because it requires a showing of a substantial limitation on a major life activity?

[¶ 33] We answer question # 2, "Yes."