The conception of philosophy presented in John Rawls’s “A Theory of Justice” (1971) still influences many approaches to political inquiry today. Rawls thought that philosophy is a theory-building activity: it takes considered judgments of reasonable persons as its starting point and aims to explicate them in terms of one or several general principles. He thus assumed that all reasonable persons would agree in their judgments, and that they would make these judgments for the same several reasons. Yet – to the dismay of many – Rawls qualified his view by stating that principles are sometimes indeterminate and that, while agreeing on general principles, reasonable persons may differ in courses of action. This qualification called into question the usefulness of political philosophy for practical politics. My dissertation is a Cambridge-style intellectual biography that covers the first fifty years of John Rawls’s life. Placing Rawls in the intellectual context of the time, I narrate the birth and development of this influential conception of philosophy. I argue that it originated in modernism, a movement in analytic philosophy that took natural science as its paragon. Beliefs in the universality of our judgment, as well as the usefulness of theory to practice, also have their origin in modernism. I then show how this modernist conception of philosophy changed along with the post-analytic turn that took place between 1940s and 1960s and was driven mainly by the Wittgensteinian tradition. Adopting many of the Wittgensteinian themes – critical of theories – Rawls came to modify his conception of theory, concluding that it guided political judgment only in a general direction. The story told, we see how Rawls’s development has parallels in attempts at theory construction in other fields, including aesthetics and political science.