This book is about ideas and beliefs rather than events and actions. It is about what medieval people thought and believed should have happened, rather than what modern historians can demonstrate actually happened. Its major “events,” strictly speaking, did not ever occur – except in the minds of those who invented them or believed them. Its actors have been called reformers, scholars, and even saints; but they may also be regarded as forgers, deceivers, and liars. Its most powerful arguments are based on sources which traditional historical methods deem either useless or hopelessly compromised; yet it shows that forgeries are good evidence for understanding medieval perceptions of the past.

Any study of medieval forgeries must consider why they mattered, then and now. Understanding why forgeries were significant in the Middle Ages involves trying to comprehend the mentality of those who wrote them. In this study, I focus on monks, who had very strong notions about truth and lying, salvation and sin, as well as the relationship of the past, present, and future. And while monks were only part of the medieval clergy, which was only a small elite in medieval society, for the period between 900 and 1150 they were some of the most prolific writers and the greatest preservers of older writings. Consequently, some surviving monastic archives are sufficiently rich to allow close analysis of the function and meaning of forgeries for contemporaries.

Understanding why scholars of the Middle Ages should care about medieval forgeries is less straightforward. Outright forgeries invented events and, therefore, were inherently counter-factual. Forgeries rewrote the past in ways intended to mislead or deceive their audiences. So such rewritings reveal what their authors wanted others to believe and the limits of credulity. For these deceptions to be convincing, they had to look right and feel right. To look right, forgeries had to be similar to genuine texts and objects they mimicked. To feel right, they relied on an audience's willingness to accept they were genuine. Thus, both proper appearance and a receptive audience were needed to deceive successfully. Form and function were inherently related and the balance of the two mattered. Formal defects might be overlooked because of a strong disposition to accept what was proposed, whereas skepticism about claims might be overcome by highly skillful presentation.