This paper explains the important topic of human rationality in Sen’s theory of justice and in social choice theory. The concept of rationality in the contemporary utilitarianism is still seen in the old sense of the maximization of self-interest. This definition is strongly criticized by Amartya Sen, who proposes a reconfiguration of this concept. The first part of the paper analyzes the notion of rationality proposed by the indian economist. Sen’s goal is to bring together ethics and economics. The outcome of the first part will be useful to understand the limits of Sen’s capabilities approach described in the second part.

Keywords: Utilitarianism; Justice; Rationality; Public reasoning; Capability approach.