In the seventeenth and early eighteenth centuries, Ottomans and Europeans shared a growing interest in empirical knowledge and unfamiliar natural substances. Duygu Yıldırım examines a largely overlooked chapter in the history of science: scholarly interactions that reshaped how knowledge was evaluated, communicated, and transformed across imperial boundaries. Drawing on multilingual textual and visual sources, she traces the movement of remedies, books, botanical illustrations, and exotic substances--including coffee, ipecacuanha, tulips, sea coconuts, and even mummies--as physicians, naturalists, merchants, and enslaved Muslims navigated unfamiliar forms of knowledge. Rather than asking who learned from whom, Yıldırım demonstrates how uncertainty itself became a productive mode of inquiry through which Ottomans and Europeans alike confronted new substances, foreign expertise, and human diversity. A History of Uncertainty offers a fresh account of cross-cultural scientific practice and makes an important contribution to global histories of science by rethinking how new knowledge, no matter how uncertain, began to matter in the early modern world.