Could human beings from different cultures use the same medicinal substances? Did botanical images have to be aesthetically appealing? In the late seventeenth century, as global trade introduced unfamiliar substances — coffee, ipecacuanha, tulip, sea coconut, even mummies — Ottomans and Europeans were at once fascinated and perplexed by the bewildering diversity of nature. Uncertain Knowledge traces this overlooked history as physicians, naturalists, merchants, enslaved Muslims, and diplomats carried remedies, substances, books, and botanical illustrations across imperial borders. Shifting the focus from who learned from whom, Duygu Yıldırım explores how both sides traded uncertainty itself by deploying it as a new mode of inquiry, a strategy for navigating foreign knowledge, and a way of managing emerging anxieties about new substances and human diversity. Built around a series of fascinating case studies grounded in new, multilingual archival research, Uncertain Knowledge revisits common assumptions about knowledge exchange and its outcomes, offering a more granular picture of scientific practice under uncertainty — and a timely reminder of the provisional, revocable, and deeply relational nature of cross-cultural encounter.