The evidential foundations of probabilistic reasoning: toward a better understanding of evidence and its usage

“The Evidential Foundations of Probabilistic Reasoning” by David A. Schum “. . . contains a collection of thoughts . . . ” (p. 1) on issues related to evidence and to inference tasks based on evidence. The study of such issues is best summarized by an expression introduced in chapter 1: “Science of Evidence.” The Science of Evidence tries “. . . to treat the study of evidence as having a life of its own . . . ” (p. 8). This perspective of examining evidence and inference with an interdisciplinary, generalist approach, is also reflected by the author David Schum himself: he is a professor of law and information technology and engineering at George Mason University. The fundamental insights he shares in this book are—unfortunately—all too often overlooked and unknown in forensic and judicial practice and research. An important feature of evidential inference is its involvement with uncertainty, and consequently its probabilistic nature. This view is held also by Schum.He acknowledges that uncertainty is a prevalent feature of reasoning tasks based on evidence, and that it attends situations of daily life but also and most prominently, legal applications: “. . . in any inference task our evidence is always incomplete, rarely conclusive, and often imprecise or vague; it comes from sources having any gradation of credibility. As a result, conclusions reached from evidence [. . . ] can only be probabilistic in nature.” (p. xiii) Unfortunately, forensic practice regularly distrusts the notion of probability because people focus on precise numbers (derived from a generous data pool). However, assigning numbers for probabilistic evidence evaluation is neither a prerequisite nor an end for analyses of evidential inference. Schum’s work is directly relevant to this aspect by demonstrating that (1) purely structural considerations on evidence and (2) adopting probabilities as numerically variable ingredients of inferences, enable us to approach numerous problems, and to explore evidential subtleties or complexities. Let us first consider (1) and then (2).