~ A Parable of Forecasting Under Model Uncertainty ~ You, the monarch, need to know when the rainy season will begin, in order to properly time the planting of the crops. You have two advisors, Pronto and Eternidad, who you trust exactly equally. You ask them both: "When will the next heavy rain occur?" Pronto says, "Three weeks from today." Eternidad says, "Ten years from today." "Good," you say. "I will begin planting the crops in a little bit over five years, the average of your two predictions." Pronto clears his throat. "If I may, Your Grace. If I am right, we should start preparing for the planting immediately. If Eternidad is right, we should expect an extreme drought, and will instead need to use the crown's resources to begin buying up food from our neighbors, for storage. These two predictions reflect totally different underlying world models, and demand two totally different and non-overlapping responses. Beginning the planting in five years is the wrong choice under either model, and guarantees that the nation will starve regardless of which of us is right." Eternidad adds: "Indeed, Your Grace. From Pronto's point of view, waiting five years to prepare is just as bad as waiting ten years – the rains will be long passed, by his model. From my perspective, likewise, we should take action now to prepare for drought. We must allocate resources today, one way or the other. What you face is not so much a problem of prediction but a decision problem with an important component of probability. Absolutely do not view our predictions as two point estimates to be averaged and aggregated – view them instead as two distinct and mutually exclusive futures that must be weighed separately to determine the best allocation of resources. Unfortunately, given the unrectifiable disagreement between Pronto and myself, the best course of action is that we do our best to make reasonable preparations for both possibilities. We should spend some fraction of our treasury o