John Rawls's *A Theory of Justice*, published in 1971, was the product of twenty years of work. In the 1950s and 60s Anglo-American political philosophy had narrowed, under the influence of British analytic philosophy, into the analysis of moral language; large normative theory had not been written for a long time. Rawls changed this with a single book.

The central thought-experiment is the 'original position'. If a community were to be re-founded, under what conditions could we all agree on its founding principles? Rawls proposes the 'veil of ignorance': imagine a starting point where the parties know nothing about themselves — their class, abilities, religion, sex, or natural tendencies. Under these conditions, what principles of justice would we choose?

Rawls advances two principles. The first principle (the principle of liberty): each person is to have the most extensive set of basic liberties compatible with the same set for everyone (speech, conscience, the vote, political participation). The second principle (the principle of inequality) is in two parts: (a) social and economic inequalities must be attached to positions open to all under conditions of genuine equality of opportunity; (b) such inequalities can only be acceptable if they are to the greatest benefit of the least-advantaged members of society (the difference principle).

The difference principle is the most disputed part. Rawls argues that a person's abilities and circumstances of birth are not their own choice — therefore not 'deserved' — and so the legitimacy of inequalities arising from them depends solely on whether they raise up those at the bottom.

From the 1970s the book became the primary philosophical defence of liberal democracy. Robert Nozick replied from the right in *Anarchy, State, and Utopia* (1974); Michael Sandel raised communitarian objections (1982); feminist philosophers pressed the question of gender. Between 1985 and 2001 Rawls reformulated parts of his thesis (*Political Liberalism*, 1993). Modern political philosophy still moves largely within the debates *A Theory of Justice* opened.