Raphael, Alex, and Clancy are lumberjacks who live next to a forest that is open to logging; in other words, anyone is free to use the forest for logging. Assume that these men are the only three lumberjacks who log in this forest and that the forest is large enough for all three lumberjacks to log intensively at the same time. Each year, the lumberjacks choose independently how many acres of trees to cut down; specifically, they choose whether to log intensively (that is, to clear-cut a section of the forest, which hurts the sustainability of the forest if enough people do it) or to log nonintensively (which does not hurt the sustainability of the forest). None of them has the ability to control how much the others log, and each lumberjack cares only about his own profitability and not about the state of the forest.
Assume that as long as no more than one lumberjack logs intensively, there are enough trees to regrow the forest. However, if two or more log intensively, the forest will become useless in the future. Of course, logging intensively earns a lumberjack more money and greater profit because he can sell more trees.