The first edition of what would become the most widely used medical reference book of the twentieth century opened not with advice but with a warning: “Memory is treacherous.” Released in 1899, Merck’s Manual of Materia Medica, later called The Merck Manual, gave voice to the growing perception of information overload, as therapeutic preparations and medical publications began to proliferate like never before. The number of available remedies meant that “the most thoroughly informed” physician was one whose “mind is overburdened.” “But a mere reminder,” The Manual reassured, “is all he needs, to make him at once master of the situation and enable him to prescribe exactly what his judgment tells him is needed for the occasion.”

It is a truism in medical practice that “patients don’t read the book”—that patients and diseases present in idiosyncratic ways. An overreliance on books, physicians have been warned, may lead one astray. Yet physicians have long depended on reference tools of all kinds, from textbooks and pocket guides to reference files and smartphone apps. Today, reference tools participate in nearly every patient encounter, as physician struggle to keep up with an ever expanding and evolving base of evidence. This research project focuses on the history of reference tools in medicine and explores the ways in which these tools have shaped how physicians think, practice, and self-identify.

The medical reference landscape in is a state of rapid change as reference tools are integrated with clinical trials, machine-learning algorithms, and electronic medical records. Choices regarding reference tools are deeply intertwined with clinical, cultural, and even sartorial norms (what pocket guide could fit into today’s favored clinical uniform, the embroidered fleece?). The demands and constraints associated with search algorithms and “prompt engineering” may engender new ways of approaching, framing, and engaging with medical data. This history of reference tools places these changes within a larger historical context, demonstrating the ways in which the physician’s proverbial “peripheral” brain is anything but.