Greene reconstructs the crucial months Meriwether Lewis spent in Philadelphia in the spring of 1803, receiving intensive scientific training in preparation for the expedition at Jefferson’s direction. The article documents Lewis’s studies with Benjamin Smith Barton (botany), Caspar Wistar (anatomy and fossils), Benjamin Rush (medicine), Robert Patterson (astronomy and navigation), and Andrew Ellicott (surveying and instruments). Greene examines the curricula and methods each mentor employed, the scientific instruments and reference materials Lewis acquired, and how this compressed education shaped the expedition’s approach to natural history observation. The article also discusses Dr. Rush’s famous list of medical questions about Native peoples and the pharmaceutical supplies Lewis assembled, including the notorious “Rush’s Thunderbolts” purgative pills. Greene argues that Lewis’s Philadelphia training, while brief, was remarkably effective and enabled scientific observations of lasting value.