As known not only by lawyers, judges, and regulators but also businesspeople, scientists and pretty much anybody who has ever had to make a decision on the basis of technical assessments of the relevant world, in any tough decision of this sort, there is conflicting evidence and opinion. Some things are known with certainty. We know for certain, for example, that the sun will rise in the east. But most important decisions are taken under conditions of uncertainty. A farmer deciding when to plant his fields with corn should consider evidence about the likelihood of a late, crop-killing frost; in deciding how to design a new product for market, a prudent business will get evidence about likely consumer demand for alternative designs at varying prices; a court deciding whether or not the defendant committed a crime will hear not only the prosecution’s evidence but also the defendant’s. Even after hearing evidence on both sides, a decision maker will not be certain about the state of the world relevant to her decision.