Since the late 19th century, the image of Alexander von Humboldt has been fractured into that of the patient and assiduous fact-gatherer, devoted to measurement and quantification, and that of the sensitive soul, awake to the unity and beauty of the landscape. Concern with emotional and aesthetic responses to the natural world was, however, central to Humboldt's precise and quantitative approach to natural history. The unity of his project may be better understood by exploring his youthful immersion in Enlightenment debates over the nature of the human mind and the possibility of rational knowledge of nature — debates which took on a special urgency during the epoch of the French Revolution. Specifically, the reforms of natural history which Humboldt proposed in the 1790s and practiced during his expedition to the Americas (1799–1804) drew on the concepts and techniques of “analysis” developed by the French Encyclopedists and refracted through German politics and philosophy. Humboldt's approach to natural history thus exemplifies the essential continuity between Enlightenment doctrines of sensation and sensibility and Romantic assertions of the unity of nature and the unique role of the naturalist in revealing that unity.