This study investigates the influence of ideas of imagination in the development of Chaucer’s poetic vision and achievement. Chaucer’s poetry was decisively marked by his concerns about the crucial mediating role played by imagination in the processes of human perception, concerns which raised troubling implications about the reliability and truth of his observations and impressions. The importance of this defining strand of Chaucer’s thinking has been critically overlooked, not least because it requires the consideration of difficult questions about the overlapping relationship of medieval science, spirituality and aesthetics. Understanding what the term imagination meant to Chaucer demands analysis of many diverse topics. This process is complicated not merely by shifts in meaning between medieval and modern usage, but also by variation in medieval understandings of imagination, and shifts in meaning across this time period. In part, these reflect changing valuations of the respect due to human individuality and creative originality. This study is based on the premise that Chaucer’s poetry is the work of an exceptionally self-aware and self-questioning artist, and that evidence of these qualities of his poetic practice was consistently, if sometimes elusively, integrated into his verse. The ways in which the term ‘imagination’ is sparingly but prominently deployed in Chaucer’s texts presents an important example of such evidence, while also exemplifying the linguistic precision through which his poetic and spiritual preoccupations were—however evasively—signaled to his readers. This book considers how his reservations about imagination shaped Chaucer’s conflicted attitude to the claims of literary authority, which underlies his supposed inability to align his work fully within this tradition.