Reconciling Bayesian Epistemology and Narration-based Approaches to Judiciary Fact-finding

Abstract

Legal probabilism (LP) claims the degrees of conviction in juridical fact-finding are to be modeled exactly the way degrees of beliefs are modeled in standard bayesian epistemology. Classical legal probabilism (CLP) adds that the conviction is justified if the credence in guilt given the evidence is above an appropriate guilt probability threshold. The views are challenged on various counts, especially by the proponents of the so-called narrative approach, on which the fact-finders' decision is the result of a dynamic interplay between competing narratives of what happened. I develop a way a bayesian epistemologist can make sense of the narrative approach. I do so by formulating a probabilistic framework for evaluating competing narrations in terms of formal explications of the informal evaluation criteria used in the narrative approach. 1 1 Legal probabilism and its troubles According to legal probabilism (LP), degrees of conviction in juridical fact-finding are to be modeled exactly the way degrees of beliefs are modeled in standard bayesian epistemology: by means of probabilistic distributions satisfying the standard axioms of probability theory. Classical legal probabilism (CLP), which originated with Bernoulli [3] adds on top of that the view according to which the criminal standard of proof beyond reasonable doubt should be equated with a certain high threshold probability of guilt (albeit, some variants of the view admit that thresholds for different cases might be different). 2 LP (and generally, the use of probabilistic methods in judiciary contexts) is criticized from various angles (see for example [18], [19], [4], [20], [14], [5], [6], [21], [17], [2], [10], [9]). The critics of LP argue that the view is blind to various phenomena that an adequate philosophical account of legal fact-finding should explain. Some of them pertain to procedural issues [17]: proceedings are back-andforth between opposing parties, cross-examination is crucial, and yet CLP seems to take no notice of this dynamics. Some have to do with reasoning methods which are not only evidence-to-hypothesis, but also hypotheses-to-evidence [21,2] and involve inference to the best explanation [6]. A better account, arguably, is one in which the proceedings are seen as an interplay of evidence and various explanations (often called narratives) presented by opposing parties [10].Accordingly, the no plausible alternative story (NPAS) theory [1] is that the courtroom is a confrontation of competing narrations offered by the defendant and by the prosecutor and the narrative to be selected should be the most plausible one. The view is conceptually plausible [7] and finds support in * The research has been funded by Research Foundation Flanders.