about the need to conserve stocks. When the fish population falls to half the maximum (fish density equal to 0.5) the marine biologist argues that stocks are too low. But the fisherman reports (accurately) there is no difficulty catching fish, so what's the problem? In all likelihood, the fisherman thinks the fish stock is actually higher than the marine biologist's estimate. The biologist is exaggerating the problem, or so it seems to someone whose livelihood depends directly on the catch. When the fish population falls to one-quarter of the maximum (fish density equal to 0.25) the marine biologist is frantic and even the fisherman is beginning to notice a reduction in the catch, down by about one-third relative to normal. That outcome, though worrying, is not obviously fatal. Perhaps with a bit more effort and luck the poor catch can be rectified, and why believe the marine biologist now, when he/she was seemingly so wrong and alarmist before? The non-linearity creates confusion in the attribution of causality – what causes what in the system – and such confusion is a typical symptom of dynamic complexity.