This paper examines the Defective Version of The Book of John Mandeville as a case study of the construction of natural knowledge in late 14th century Britain. I examine the Mandeville author’s discussions of flora and fauna, particularly marvelous creatures, to explore the standards of evidence in medieval discussions of nature. As one of the most widely read works of its age The Book offers us an interesting opportunity to explore the presentation of natural information to a non-scholastic, vernacular, reading public. The Book of John Mandeville is a travel narrative purporting to be written by the English knight John Mandeville recording his experiences on pilgrimage to Jerusalem and his journeys into the Far East. The world he travels through is one full of fabulous creatures and strange races, from men with their faces in their chest to lambs born from fruit. Through this account of his marvelous journey the author constructs a complex, and to many of his readers convincing, vision of the world using knowledge claims drawn from the scholarly authority of encyclopedias, histories and religious works combined with the eyewitness accounts of travelers. The world he describes has both natural and spiritual significance; something scholars sometimes refer to as natural and spiritual geographies. In this paper I argue that these two geographies––distinct but intertwined understandings of the world––provide differing contexts, and through them differing standards, for creating knowledge claims. The Mandeville author’s approach shifts fluidly between contexts, moving from one kind of evidence to another or employing multiple types of evidence to establish a point that functions within multiple contexts. The Book allows us to examine how claims based on authority, eyewitness and common knowledge interact; shifting their epistemic value in relation to the subject’s position in these intertwined spiritual and natural geographies. By exploring these themes in the Defective Version of The Book of John Mandeville I will begin to unpack some of the practices of evidence use associated with the presentation of natural information to a late 14th century audience.