The Epistemology of Admissibility: Why Even Good Philosophy of Science Would Not Make for Good Philosophy of Evidence

Brian Leiter* I. INTRODUCTION In its 1923 decision in Frye u. United States,l the United States Court of Appeals for the District of Columbia set out what was, for seventy years, the most influential test for the admissibility of scientific evidence in federal court. In Frye, the question was whether the results of a lie detector test were admissible on behalf of the defense. The Court of Appeals agreed with the trial court that such evidence was inadmissible, famously holding, that scientific evidence "must be sufficiently established to have gained general acceptance in the particular field in which it belongs."2 In 1993, the United States Supreme Court ended Frye's reign of influence with its decision in Daubert v. Merrell Dow Pharmaceuticals, Inc.3 Holding that Federal Rule of Evidence 702,4 governing the admissibility of scientific evidence, did not codify Frye's "general acceptance" test,5 the Court went on to say that the key question was whether any proffered piece of evidence constituted "scientific knowledge" within the meaning of the Rule.' The Court then enumerated a nonexclusive list of factors for courts to consider in assessing whether proffered evidence constitutes "scientific knowledge": for example, whether the theory on which the evidence is based is "falsifiable" (in Karl Popper's sense);' whether "the theory or technique has been subjected to peer review"; and whether the theory enjoys "general acceptance."' Since Daubert, trial judges must now weigh a complex set of philosophical and methodological factors in deciding upon the admissibility of proffered scientific evidence, rather than falling back upon the simple proxy of "general acceptance." The Supreme Court's repudiation of Frye as the exclusive test for admissibility of scientific evidence has already generated substantial scholarly literature.9 From a philosophical standpoint, the most interesting recent discussion has been Heidi Feldman's argument that Daubert's rejection of Frye and its broadening of the criteria for admissibility serve to bring the Federal Rules of Evidence more in line with what Feldman dubs "revised empiricist" philosophy of science, and thus with actual scientific practice.lo Feldman commends this move toward a "more scientific approach to admissibility,"" especially because its consequence will be, according to Feldman, a greater recognition of scientific uncertainty, especially in the area of mass tort litigation.l2 In this Article, I take issue with Professor Feldman's argument. For even if "revised empiricist" philosophy of science were the dominant, or even the correct, philosophy of science, there would be no reason to think admissibility standards ought to conform to it: Feldman seems to confuse the philosopher's question, "What is the best account of scientific method?" with the lawyer's question, "What is the best criterion for judges to use in deciding the admissibility of scientific evidence?" Yet the philosopher can provide the lawyer with good epistemological reasons for keeping these questions separate-reasons that Feldman ignores or downplays. Philosophy of science and the rules of evidence are both concerned with "epistemic norms": norms for how scientists should form beliefs in the former case; norms for how juries should form beliefs about disputed matters of fact in the latter case. A central theme of contemporary "naturalized" epistemology has been the need to tailor our normative advice about belief formation to the realities of how epistemic processes-processes for the acquisition of knowledge-work, whether they involve individual mechanisms (e.g. perception) or social mechanisms (e.g. trials).l3 In particular, norms for belief formation must be sensitive to the epistemic limits of would-be knowers: that is, the handicaps-intellectual, cognitive, temporal, material-that all real knowers operate under. If scientific knowers differ, as of course they do, from the "knowers" that comprise juries and that sit on the bench, then it should be surprising, from the perspective of naturalized epistemology, that the same norms for belief-formation should apply to both groups. …