Chapter 3

### Introduction

Approaches to scientific inquiry—including evidence-based medicine (EBM)—depend on how one views the nature of knowledge as evidence, how it should be acquired, and how it should be applied (epistemology). The goal of this chapter is to make the EBM perspective on this issue—seldom clearly articulated—clearer. In this discussion, we highlight the 3 key principles of EBM (see Chapter 2, What Is Evidence-Based Medicine?).

### What is Evidence?

Philosophers do not agree about the definition of evidence.1 The concept is further complicated by language: different languages translate “evidence” in different ways. Some consider it synonymous with “proof,” “fact,” or “knowledge.”

Many philosophers define evidence as “grounds for belief.”1 In this view, evidence provides support for a contention or belief.1,2 Thus, evidence, which can constitute measurements of observable events or reports of sensory states (eg, pain, fatigue, and nausea), serves to enhance (or diminish) our confidence in some particular claim.1,2 In this view, evidence is not tied to any specific theory of knowledge.3 Rather, evidence represents the necessary basis for effective problem solving and decision making.

Evidence-based medicine suggests a broad definition of evidence: any empirical observation or report of a symptom or mental state constitutes potential evidence, whether systematically collected or not. Thus, the unsystematic observations of individual clinicians constitute a source of evidence, a patient's report of feeling tiredness or pain would represent a second source of evidence, physiologic experiments constitute another source, and clinical trial results constitute a fourth.

### Claims Should Consider All of the Most Credible Evidence

Most philosophers contend that the concept of evidence is inseparable from the concept of justification (ie, what is justified or reasonable to believe depends entirely on the trustworthiness of one's evidence). This view is known as evidentialism.4 Frequently, however, evidence is inconsistent (ie, points to different conclusions). Under these circumstances, people tend to select evidence that favors their particular views, which often vary, sometimes markedly.

Evidence-based medicine, while acknowledging that interpretation of evidence is inevitably subjective, is consistent with philosophical views that endorse a central role of evidence as the basis of generating agreement among rational observers.1,2 Philosophers also have found that the pursuit of truth is best accomplished by examining the totality of evidence5 rather than selecting evidence that favors a particular view.6 This position is a core principle of EBM, denoted herein as the first principle of EBM (Box 3-1).2 In practice, this means that our inferences (and decisions) are best informed by systematic reviews (ie, syntheses of the totality of relevant high-quality evidence) of the effects of health care interventions as epitomized by the work of the Cochrane Collaboration.7

BOX 3-1

Epistemologic Principles of Evidence-Based Medicine

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