James Parkinson's Essay on the Shaking Palsy, published in 1817, represents a landmark in the development of writing about neurologic disorders. Parkinson was an astute clinician-investigator, and his wide scientific interests and ideas on social advancement in many ways typified the spirit of the Age of Enlightenment. Our commentary on the text of his essay identifies important sources of its originality: the particular way in which Parkinson collected and categorized clinical material, his use of a field neurology method to identify affected individuals, and his skills as a narrative writer. Although the essay belongs to an older tradition of disease classification, it also anticipates the modern neurologist's reliance on accurate clinical description and natural history in establishing a diagnosis.