Halo 3 - How Microsoft Labs Invented a New Science of Play

Creating a game, in contrast, is like a combination of architecture — constructing environments that influence the behavior of people inside them — and designing a new sport. Gamemakers have to devise a system of rules and equipment that gives players a few basic goals and then allows them to find their own ways of achieving those goals. The flow comes from constantly discovering innovative ways to solve these open-ended problems.

Halo 3: How Microsoft Labs Invented a New Science of Play