Discussing world-wide justice is messy not merely because of its scope, the amount of disagreement about the issues, cultural differences, uncertainty about political possibilities or the outcome of present developments. There are more elementary and fundamental difficulties, argues Janna Thompson. For example, how should we reason about global justice? Can we reason in the same way as we reason about justice in national societies, or is a different approach required? Should we aim to determine the principles of justice for global society, or something else?