Introduction

Mary and Jim are roommates. They share an apartment with separate bedrooms and common living and eating areas. Both suffer during the winter from dry air – a problem that can be solved by installing a humidifier that will add moist air to the entire apartment. The cost of a humidifier is significant for both Mary and Jim. One day, Mary installs a humidifier and asks Jim to pay half the cost. Does Jim have an obligation to pay?

The literature on the duty of fair play is filled with variations on this simple example. In fact, the inventiveness of theorists in creating hypothetical cases to test intuitions about the duty of fair play is matched only by – and is probably a reflection of – the inability to reach a consensus about the underlying theory that grounds such a duty. It is as if, to borrow from Rawls's concepts of reflective equilibrium and considered judgments, explanations of the duty of fair play are so confused or lacking in consensus at the level of general theory that most of the work continues to be done at the level of intuition. Examples and counterexamples parade for inspection in the hope that judgments about whether a duty exists in a particular case can be connected to a general theory, thus making the judgments more secure and considered.