As the previous chapters have illustrated, it is often quite easy to come up with physical models that determine the effects that result from various causes — we know how image intensity is determined, for example. The difficulty is that effects could have come from various causes and we would like to know which — for example, is the image dark because the light level is low, or because the surface has low albedo? Ideally, we should like to take our measurements and determine a reasonable description of the world that generated them. Accounting for uncertainty is a crucial component of this process, because of the ambiguity of our measurements. Our process of accounting needs to take into account reasonable preferences about the state of the world — for example, it is less common to see very dark surfaces under very bright lights than it is to see a range of albedoes under a reasonably bright light.