As the historian Robert Darnton once reminded, historical “raw material” doesn't exist. It is, he said, “all cooked.” Every piece of paper, every photo, every song, every bit of information humans record can only be understood in context, and historians face the daunting task of creating it. They are judges, not just collectors, of knowledge, and, as such, must remind themselves regularly of Darnton's warning that every source embodies a rhetorical convention, argues ahidden agenda, and must be scanned between the lines.

Moreover, most historical evidence, even during the technology-driven twentieth century, has disappeared without a trace. However, the fragments left that enable analysis of the ways Americans governed their young between 1900 and 2000 fill so many shelves and so many archival boxes that anyone committed to reading them all will invariably fail. It would take a century. This book, like any work of history, has argued from documents. It has not assembled all possible information on its subject. Information, “by its very nature, is bottomless.”

Therefore, historical writing demands a wise detective's wary recognition that all the data unearthed can never exactly replicate what occurred. Yet, detectives must eventually end their investigations, hoping the bits of fiber, the trace of blood, the shard of glass led them to the right conclusions.