 The problem that contractarians face is to show how such an agreement is possible. If we were to ask people, in the real world, what principles they would prefer to live under, they are likely to start from a position of quite radical disagreement, given their interests and their beliefs. Some might even be willing to endorse slavery, if they were fairly certain that they would not end up as slaves themselves, or if they were sado-masochists who viewed the humiliations inflicted on slaves in a positive light. So in order to show how agreement could be achieved, contractarians have to model the contracting parties in a particular way, either by limiting what they are allowed to know about themselves or about the future, or by attributing to them certain motivations while excluding others. Since the modelling can be done differently, we have a family of contractarian theories of justice, three of whose most important members are the theories of Gauthier, Rawls and Scanlon.