Bayesian reasoning and the prior in court: not legally normative but unavoidable

We introduce Bayesian reasoning in court not as a toolbox for doing computations, but as a way to assess evidence in a case. We argue that Bayesian reasoning comes naturally, even when the findings in a case cannot readily be translated into numbers. Not having numbers at one's disposal is not an obstacle to use Bayesian reasoning. Although we present a coherent and complete view, we focus on the prior, since that seems to be the most problematic part of Bayesian reasoning. We explain that attempts to numerically express the prior fail in general, but also that a prior is necessary and cannot be dispensed with. Indeed, we explain in detail why decision-making should not be based on likelihood ratios alone. We next discuss two of the most delicate questions around the prior: (1) the possible conflict with the presumption of innocence, and (2) the idea that unwanted personal conviction (like racism) might enter the decision procedure via the prior. We conclude that these alleged problems are not problematic after all, and we carefully explain this position.