The concept of written legal rules—of the law itself—assumes that their content is fixed and ascertainable. The rule of law likewise depends on citizens having advance notice of what they can and cannot do, pursuant to clear, knowable directives. Legal scholars expend enormous energy pontificating about the appropriate techniques judges should apply in the course of constitutional interpretation: textualism, originalism, and so forth. Libertarian theorists argue strenuously that judges must be given greater authority—through “judicial engagement”—over the political branches. Each day, lawyers across the country trot off to court, briefs in hand, hoping to convince a black-robed judge–enthroned behind a raised, magisterial bench—that the relevant legal rules, properly construed, compel a ruling in favor of their client.