The paper examines some recent developments of Amartya Sen's thinking. Amartya Sen's analysed deeply in his research work the relationship between ethics and economics. The Idea of Justice (2009) is a synthesis and a further development of his study in the field of social and economic welfare. He provides criteria of reason in order to reduce injustice in every human society. Accordingly he aims not to define what he calls criteria of transcendental justice, but instead rules of human behaviour that could be reached and established through a public debate in liberal and democratic societies. Sen's thinking is developed in the rationalist tradition of the Enlightenment, but following the Borda - de Condorcet approach, with the consequences of this choice. The paper analyses method, goals and results of Sen's enterprise, and discusses some issues that seem unsolved.