I have articulated a two-part theory of the burden of proof, applicable in all conventional forms of trial, whether inquisitorial or adversarial: (1) it is the duty of the court to ensure, through a variety of procedural devices, that the evidence is reasonably complete, that is, that Keynesian weight has been practically optimized, and (2) it is the duty of the fact-finder to determine whether the admissible evidence, assessed in its appropriate context, satisfies a threshold criterion of epistemic warrant, one that is determined by a public assessment of the relative benefits and costs of accurate and inaccurate verdicts. Ordinarily, the fact-finder's determination does not involve assessing Keynesian weight or the relative completeness of the evidence. Several scholars have suggested that a third idea is important in knowing whether the burden of proof has been satisfied. This idea is often expressed as “tenacity of belief.” There are several versions of this idea; it would be more accurate to say that it is a family of related ideas. In this book, I cannot examine every permutation of the idea that is available, but I will try to identify significant contributions along this line sufficient in number and variety to make clear their relatedness. I will argue that, provided that one takes appropriate account of discriminatory power and Keynesian weight, the idea of tenacity of belief is, in the end, a false lead. Whatever it has to contribute to our understanding of burdens of proof is already embedded in the ideas of discriminatory power and Keynesian weight.

“Tenacity of belief” refers to the degree of resistance a person has to giving up the belief. As with discriminatory power and Keynesian weight, tenacity of belief is a gradational attribute associated with a person's epistemic assessments. It can be applied to beliefs that are conceived as gradational or to beliefs that are conceived as categorical.