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MICHIGAN v. EPA THOMAS, J., concurring

implementation by an agency, understood that the ambiguity would be resolved, first and foremost, by the agency, and desired the agency (rather than the courts) to possess whatever degree of discretion the ambiguity allows.” Smiley v. Citibank (South Dakota), N. A., 517 U. S. 735, 740–741 (1996). We most often describe Congress’ supposed choice to leave matters to agency discretion as an allocation of interpretive authority. See, e.g., National Cable & Telecommunications Assn. v. Brand X Internet Services, 545 U. S. 967, 983 (2005) (referring to the agency as “the authoritative interpreter (within the limits of reason) of [ambiguous] statutes”). But we sometimes treat that discretion as though it were a form of legislative power. See, e.g., United States v. Mead Corp., 533 U. S. 218, 229 (2001) (noting that the agency “speak[s] with the force of law when it addresses ambiguity in the statute or fills a space in the enacted law” even when “ ‘Congress did not actually have an intent’ as to a particular result”). Either way, Chevron deference raises serious separationof-powers questions. As I have explained elsewhere, “[T]he judicial power, as originally understood, requires a court to exercise its independent judgment in interpreting and expounding upon the laws.” Perez v. Mortgage Bankers Assn., 575 U. S. ___, ___ (2015) (opinion concurring in judgment) (slip op., at 8). Interpreting federal statutes—including ambiguous ones administered by an agency—“calls for that exercise of independent judgment.” Id., at ___ (slip op., at 12). Chevron deference precludes judges from exercising that judgment, forcing them to abandon what they believe is “the best reading of an ambiguous statute” in favor of an agency’s construction. Brand X, supra, at 983. It thus wrests from Courts the ultimate interpretative authority to “say what the law is,” Marbury v. Madison, 1 Cranch 137, 177 (1803), and hands it over to the Executive. See Brand X, supra, at 983 (noting that the judicial construc-