Abstract: This essay attempts to recover a line of argument, developed by James Fitzjames Stephen and Justice Holmes, that describes majority rule as an irresistible force of nature. Majority rule has political and psychological force independent of its intrinsic merits. That force arises from a range of mechanisms, including the threat of majoritarian violence, the simplicity and focal-point character of majority rule, and political envy. Where the force of majority rule is sufficiently powerful, Stephen argues, majority rule is simply a political constraint, whatever its merits from an ideal point of view. Holmes then urges a least-cost principle: where majorities will inevitably get their way, law should at least ensure that they get their way efficiently - in a manner that minimizes total social cost. This principle has implications for the extension of the suffrage, legislative voting rules, delegation to bureaucracies, criminal law, free speech law, and the law and politics of emergency powers. The conclusion argues for an explanatory approach to social choice theory, as opposed to normative social choice, and for a second-best approach to normative social choice, in which the analyst assumes realistic political constraints.