 Ethical and political questions regarding marriage are sometimes answered by appeal to the definition of marriage. But the historical and cultural variation in marital practices has prompted some philosophers to argue that marriage is a ‘family resemblance’ concept, with no essential purpose or structure (Wasserstrom 1974; see also the interesting discussion of whether the many different practices identified by anthropologists as marriage should be counted as marriages in Nolan, forthcoming). If marriage has no essential features, then one cannot appeal to definition to justify particular legal or moral obligations. For instance, if monogamy is not an essential feature of marriage, then one cannot appeal to the definition of marriage to justify a requirement that legal marriage be monogamous. To a certain extent, the point that actual legal or social definitions cannot settle the question of what features marriage should have is just. First, 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). Second, appeal to definition may be uninformative: for example, legal definitions are sometimes circular, defining marriage in terms of spouses and spouses in terms of marriage (Mohr 2005, 57). Third, appeal to an existing definition in the context of debate over what the law of marriage, or its moral obligations, should be risks begging the question: in debate over same-sex marriage, for example, appeal to the current legal definition begs the normative question of what the law should be. However, this point also tells against the argument for the family resemblance view of marriage, as the variation of marital forms in practice does not preclude the existence of a normatively ideal form. Thus, philosophers who defend an essentialist definition of marriage offer normative definitions, which appeal to fundamental ethical or political principles. Defining marriage must depend on, rather than precede, ethical and political inquiry.