Apart from his summarized autobiography, Sen’s wide command of economics and philosophy is described in this chapter’s Part I. He brought philosophical notions back into economics, especially those concerning liberal political philosophy. Sen confronted Kenneth Arrow’s social choice theory with critical observations. Also John Rawls could not escape Sen’s sharp analysis of Rawls’s theory of justice. Sen also widened the scope of economics in joining development economists, arguing that what is happening in developing countries should form an integral part of economics. Part II describes Rawls’s theory of justice, followed by Sen’s critical analysis and his alternative capability theory. The same applies to Arrow’s social choice theory, including his Impossibility Theorem. Sen arrived at a more positive conclusion than Arrow on this theorem. As Arrow and Rawls revived contractarian theory, the Annexe about social contract theories, ranging from Thomas Hobbes to Jean-Jacques Rousseau, concludes Part II.