Decades into its existence as a foundational aspect of modern political and economic life, the welfare state has become a political cudgel, used to assign blame for ballooning national debt and tout the need for personal responsibility. At the same time, it affects nearly every citizen and permeates daily life--in the form of pension, disability, and unemployment benefits, healthcare and parental leave policies, and more. At the core of that disjunction is the question of how we as a society decide who should get what benefits--and how much we are willing to pay to do so. Probable Justice traces a history of social insurance from the eighteenth century to today, from the earliest ideas of social accountability through the advanced welfare state of collective responsibility and risk. At the heart of Rachel Z. Friedman's investigation is a study of how probability theory allows social insurance systems to flexibly measure risk and distribute coverage. The political genius of social insurance, Friedman shows, is that it allows for various accommodations of needs, risks, financing, and political aims--and thereby promotes security and fairness for citizens of liberal democracies.