The Eureka Day School in Berkeley, California, is a bastion of progressive ideals: representation, gender identity, social justice. In weekly meetings Eureka Day’s five board members develop and update policy to preserve this culture of inclusivity, reaching decisions only by consensus. But when a mumps outbreak threatens the Eureka community, facts become subjective and every solution divisive, leaving the school’s leadership to confront the central question of our time: How do you build consensus when no one can agree on truth?