Introduction

“Uncertainty” is a capacious term, used to encompass a multiplicity of concepts. Uncertainty may arise because of incomplete information – what will be the U. S. defense budget in the year 2050? – or because of disagreement between information sources – what was the 1987 Soviet defense budget? Uncertainty may arise from linguistic imprecision – what exactly is meant by “The river is wide”? It may refer to variability – what is the flow rate of the Ohio River? Uncertainty may be about a quantity – the slope of a linear dose-response function – or about the structure of a model – the shape of a dose-response function. Even where we have complete information in principle, we may be uncertain because of simplifications and approximations introduced to make analyzing the information cognitively or computationally more tractable. As well as being uncertain about what is the case in the external world, we may be uncertain about what we like, that is about our preferences, and uncertain about what to do about it, that is, about our decisions. Very possibly, we may even be uncertain about our degree of uncertainty. The variety of types and sources of uncertainty, along with the lack of agreed terminology, can generate considerable confusion.