Led by G. A. Cohen (a moral philosopher by training), they argued that a Marxist theory of justice had to focus on egalitarianism. For Cohen, this meant an engagement with moral and political philosophy in order to demonstrate the injustice of market exchange, and the construction of an appropriate egalitarian metric. This argument is pursued in Cohen's books, Self-Ownership, Freedom and Equality (1995) and If You're an Egalitarian How Come You're So Rich? (2000b).