 However, if marriage is essentially different-sex, excluding same-sex couples is not unequal treatment; same-sex relationships simply do not qualify as marriages. One case that marriage is essentially different-sex invokes linguistic definition: marriage is by definition different-sex, just as a bachelor is by definition an unmarried man (Stainton, cited in Mercier 2001). But this confuses meaning and reference. Past applications of a term need not yield necessary and sufficient criteria for applying it: ‘marriage’, like ‘citizen’, may be extended to new cases without thereby changing its meaning (Mercier 2001). As noted above, appeal to past definition begs the question of what the legal definition should be (Stivers and Valls 2007).