what marriage is.
Underlying our equal protection jurisprudence is the central notion that "[a] classification 'must be reasonable, not arbitrary, and must rest upon some ground of difference having a fair and substantial relation to the object of the legislation, so that all persons similarly circumstanced shall be treated alike.'" Reed v. Reed, 404 U.S. 71, 76 (1971) (quoting F. S. Royter Guano Co. v. Virginia, 253 U.S. 412, 415 (1920)). The modern tiers of scrutiny—on which Windsor and the United States rely so heavily—are a heuristic to help judges determine when classifications have that "fair and substantial relation to the object of the legislation." Reed, supra, at 76.
So, for example, those classifications subject to strict scrutiny—i.e., classifications that must be "narrowly tailored" to achieve a "compelling" government interest, Parents Involved in Community Schools v. Seattle School Dist. No. 1, 551 U.S. 701, 720 (2007) (internal quotation marks omitted)—are those that are "so seldom relevant to the achievement of any legitimate state interest that laws grounded in such considerations are deemed to reflect prejudice and antipathy." Cleburne v. Cleburne Living Center, Inc., 473 U.S. 432, 440 (1985); cf. id., at 452–453 (Stevens, J., concurring) ("It would be utterly irrational to limit the franchise on the basis of height or weight; it is equally invalid to limit it on the basis of skin color. None of these attributes has any bearing at all on the citizen's willingness or ability to exercise that civil right").
In contrast, those characteristics subject to so-called intermediate scrutiny—i.e., those classifications that must be "'substantially related'" to the achievement of "important governmental objective[s]," United States v. Virginia, 518 U.S. 515, 524 (1996); id., at 567 (Scalia, J., dissenting)—are those that are sometimes relevant considerations to be taken into account by legislators, but "generally provid[e] no sensible ground for different treat-
