Thomas Bustamante

At the outset of The Proof, in a passage that grasps the spirit of the book, Frederick Schauer writes:

It would be nice if there were world peace and nonfat bacon, but wishing won’t make it so. … Leaving to others questions about how we or the government ought to act, this book is an attempt to provide some insight into how we do – and, yes, should – confront the factual questions and controversies that are all around us. (P. 4.)

Schauer’s intriguing book analyzes a vast range of subjects related to the practice of giving and evaluating evidence, covering topics such as probability analysis, burdens of proof, statistics, testimony, lie detection, expert evidence, and scientific evidence in criminal law. But only two of the topics covered in the book will occupy my attention now: his hybrid theory of evidence, and his concern about motivated reasoning.

Let us begin with the general account of evidence provided in chapters one and two. A central concern of the book is the need to distinguish between “empirical reality” and “what some or many people prefer or wish that empirical reality to be.” (P. 1.) Evidence matters only for those who want to make sense of this distinction, and who worry about getting the truth of some matter. Evidence is what provides “a justification, or warrant, as philosophers are prone to put it, for believing that something is true – or false.” (Pp. 4-5.) It is, therefore, the “prerequisite for judgments of truth (and falsity)” about anything. (P. 5.)

But how should we go about to discover the truth of some disputable fact? Schauer’s general answer is based on a Bayesian account of probability and inductive reasoning. “The very idea of evidence,” he argues, “is about inductive reasoning and thus about probability.” (P. 13.) Contrary to the hope of finding a direct and infallible way of grasping the factual reality, Schauer argues that “evidence comes in degrees.” He mentions a well-known adage from Dr. Theodore Woodward, a famous researcher in medical studies from the 1940’s: “When you hear hoofbeats, think horses, not zebras.” The point of the adage, Schauer argues, “is that horses are far more common than zebras, at least if we are not in a zoo or on the African savanna”. (P. 15.)

Schauer avoids spending much time on discussing the formal details of Bayes’s theorem, but he emphasizes its point that evidentiary reasoning is incremental: “Nonformally, Bayes’ theorem is about the way in which additional evidence incrementally (or serially) contributes to some conclusion.” (P. 24.) We begin with an estimate or some ordinary assumption about the likelihood of some conclusion, which on Bayesian terminology is called prior probability or, for short, the prior. (P. 24.) As people find or are given further evidence, “they consider each new piece of evidence and readjust the probability of their earlier conclusion upward or downward to produce the posterior probability.” (P. 25.) All evidence, “or at least almost all evidence” has, thus, a double aspect: “It is typically based on other evidence, and it is also evidence of something else.” (P. 24.)

One of the central ideas of this model, as Schauer emphasizes, is that that “beliefs come in degrees” (P. 28), and consequently we should think of evidence as being capable of being “stronger than” (or “weaker than”) other evidence with which it may happen to compete. At this point, Schauer’s Bayesian model faces an objection. “On a pure Bayesian approach, what we do when it is time to reach a conclusion is to see where we are at that stage in the process of Bayesian updating. Each incremental item of evidence adjusts the probabilities, and at the moment of decision…we make a decision based on the probabilities at that point.” (P. 31.) But one can argue that this approach is too narrow, as the supporters of the “inference to the best explanation” model suggest. According to this approach, developed by Gilbert Harman and further enhanced by Peter Lipton, “the evidence for (or against) some conclusion is not evaluated incrementally. Instead, all of the evidence is evaluated holistically, with the aim of seeing which explanation best explains what we have to that point obtained.” (P. 31.)

Schauer replies, however, that there is no inconsistency between the incremental approach defended by Bayes and the holistic approach defended by Harman and Lipton, and attempts to develop a hybrid model of evidence assessment that incorporates insights from both accounts. It conceives our reasoning process as comprising two stages: an incremental step, in which we assess the facts in order to gather evidence for or against a certain conclusion; and an inferential step, in which we synthesize the conclusions in the previous step to integrate our findings with the bulk of the evidence and our experience as a whole. (Pp. 32-33.)

Schauer’s solution enables us to see that there is something to learn from both approaches. While the first approach (incrementalism) does the work of collecting and gathering the inferences, the second approach (holism) does the work of processing them and making sense of the knowledge we accumulated, explaining the inferential relations between the evidence we collected and the body of experience we possess. It is only at this second stage that our assessment of evidence is complete, since it is only at this phase that we can make sense of how our perception and our inquiries fit in the body of knowledge we accumulated and in the whole body of experiences we have.

We can move now to the second issue that I want to discuss in Schauer’s book. The most persistent concern in Schauer’s book is the problem of constraining our own perception and preventing our cognitive mistakes. Schauer is worried about the destructive potential of what he described, following an influential literature on psychology and human behavior, as the problem of motivated reasoning. This problem emerges because “although most of the factual world successfully resists adapting to our preferences, often, and perhaps surprisingly, our perception of that factual world does conform to our preference” (P. 227), leading us to perceive only the reality that we would like to be.

The stronger the preferences of the observer and the greater the consequences of the assessment, the more serious this problem turns out to be. Motivated reasoning occurs not only when we are discussing moral judgments and controversial issues about thick ethical concepts. It appears also in every kind of disagreement about evidence and factual assessments, including our disagreement about “hard, plain facts” like the fact that Elvis is dead, or the fact that the Earth is round and revolves around the sun, or the fact that Biden won the American election of 2020, or the fact that vaccines are effective against COVID-19. (Pp. 226-28.) The basic idea of motivated reasoning, as Schauer explains, “is that the receipt and evaluation of evidence…is heavily influenced by normative preferences about what it would be good for the evidence to show.” (P. 228.)

Why do people look at reality, including when there is a vast amount of evidence for a simple fact, and see only what they want to see? Schauer has a specific chapter about this question, where he discusses at length the possibility of mistaken identification in the case of eyewitness or first-person participants in certain events. (Pp. 129-44.)

He gives reasons for skepticism about the distinction, which is widely accepted among lawyers in the civil law tradition, between “direct” and “indirect” (or “circumstantial”) evidence for a certain fact. In response to this claim for a special status for direct evidence, Schauer aptly demonstrates that “the law wisely knows no such category” like circumstantial evidence: “That a defendant with motive to kill a victim was in the vicinity at the time and place of the murder may be stronger evidence than an eyewitness identification by a witness with poor eyesight from a great distance on a foggy night, and stronger than a partial fingerprint taken by an inexperienced police investigator.” (P. 245.) He takes, however, a step further than this conclusion, revealing a deeper problem in the ordinary preference of lawyers and some legal scholars for “direct” evidence: the very attempt to distinguish between direct and indirect evidence is based on an unjustified preference for immediate sensorial perception over articulated inferences and an uncritical belief that a “first-person experience” provides a more reliable or better evidence for a certain fact. (P. 136.)

Outside legal discourse, for instance, it is “a tedious consistency” to find statements that claim some sort of special epistemic authority for the observers who bear some personal feature or special relationship to a set of facts. As Schauer reports in his readings of long series of letters to the editors of newspapers with wide circulation, ‘a remarkably high percentage of them begin in the same way, with an “As a…”: “‘As a retired firefighter,…”’ ‘As a college student,…’ As a recent immigrant,…’ ‘As a descendant of grandparents who died in the 1918 influenza epidemic,…’ And so on and on and on. (P. 136.)

The point about the “as a” condition, in these sentences, is to raise a claim that one’s assessment is “more reliable” or “better evidence” because of the letter writer’s “first-person experience.” (P. 136.) Nonetheless, this claim to some “special testimonial, and thus evidential, weight to participants solely because they were participants” is often undermined. “It is not clear that simply being a participant is necessarily a source of greater knowledge or greater insight. For one thing, being there can be distorting as well as enlightening.” (P. 137.) In Trump’s second impeachment trial, for instance, the allegation of some members of the US Congress who “saw little need for witness or other evidence,” because “they were there” and would be better off by delivering judgments based on their “own experience,” for instance, was described by Schauer as a particularly apt example. (P. 130.)

Schauer’s overall argument about evidence is based, in fact, on the opposite view. For his hybrid model of evidence assessment to work, one must be able to rely on a background system of assumptions, commitments, previous knowledge, data, and interpretations which allows us to trust in certain generalizations about how events can be causally related and how our experience can be ordinarily interpreted.

Our reliance on previous experience depends, therefore, on these generalizations, which play the role of grounding inferences in inductive reasoning, according to the same pattern of “ordinary” or “common inferences” like the inference that a noise from the hoofbeats near a farm comes from the neighbor’s horses, rather than from a large number of zebras escaping from a zoo that is dozens of miles away. Generalizations produce a ground for inferences from ordinary assumptions that constitute an “essential feature of our reasoning process.” They provide a focus for our evidence assessments, and establish the ways in which an alleged piece of evidence becomes relevant in any given case. (P. 30.)

Nonetheless, Schauer believes that our generalizations should not be based only on our own perception. While reliance on rules, past experiences, scientific discoveries, common sense assumptions, regularities, previous evidence and perhaps other materials that constitute our body of knowledge and experience is crucial, Schauer shows us in the last chapter of the book that there is some value in distrusting something that our “natural” or “initial” impulse would be to treat as infallible: our own perception.

Given the prominence of motivated reasoning, the mechanisms of which are the subject of detailed analysis in the final chapter of the book, Schauer is inviting us to distrust ourselves, to look for an external ground in our evidence assessments, one that reaches out for empirical evidence and verifiable inferences, instead of feelings, senses, perception, and intuition. Perhaps this warning against the failures of human perception, especially in the case of first-person reasoning about particular facts, is the greatest lesson we can take from the book. We need rules of evidence, just like we need science, legal process, and a vast range of procedures through which we test our own convictions in light of public assessment of others who have the power and the responsibility of judging them, in order to be able to trust ourselves.

The elucidation of this value (of distrusting ourselves) is a great achievement. The awareness of the possibility of motivated reasoning, including in the case of our own reasoning, and of the limits of human rationality, is a prerequisite for any kind of reasoning about a factual inquiry, whether this inquiry is performed in legal settings or in political ones; in fact, it is a prerequisite for rationality whether we are talking about sports, science, journalism, weather prediction, philosophy, history, or, like Schauer puts in the title of his fascinating book, “everything else.”