In 2009, Sen published a book called The Idea of Justice .  Based on his previous work in welfare economics and social choice theory, but also on his philosophical thoughts, he presented his own theory of justice that he meant to be an alternative to the influential modern theories of justice of John Rawls or John Harsanyi . In opposition to Rawls but also earlier justice theoreticians Immanuel Kant , Jean-Jacques Rousseau or David Hume , and inspired by the philosophical works of Adam Smith and Mary Wollstonecraft , Sen developed a theory that is both comparative and realizations-oriented (instead of being transcendental and institutional). However, he still regards institutions and processes as being important. As an alternative to Rawls's veil of ignorance , Sen chose the thought experiment of an impartial spectator as the basis of his theory of justice. He also stressed the importance of public discussion (understanding democracy in the sense of John Stuart Mill ) and a focus on people's capabilities (an approach that he had co-developed), including the notion of universal human rights, in evaluating various states with regard to justice.
Peter Townsend transformed the conception of poverty, viewing it not simply as lack of income but as the configuration of the economic conditions that prevent people from being full members of the society (Townsend, 1979;  Ferragina et al. 2016  ). Poverty reduces the ability of people to participate in society, effectively denying them full citizenship (as suggested by . Marshall ). Given that there are no universal principles by which to determine the minimum threshold of participation equating to full membership of society, Townsend argued that the appropriate measure would necessarily be relative to any particular cultural context. He suggested that in each society there should be an empirically determinable 'breakpoint' within the income distribution below which participation of individuals collapses, providing a scientific basis for fixing a poverty line and determining the extent of poverty (Ferragina et al. 2016  ).