An argument has erupted in recent years over the authorship of A short tract on first principles, a manuscript treatise traditionally regarded as the first philosophical work of Hobbes. Some have denied that it was Hobbes 's work, while others have insisted that it is. Among rival candidates, the prime suspect is Robert Payne, chaplain to the Cavendish family of Welbeck Abbey. This article offers a fresh assessment of the evidence for authorship by examining the manuscript and its contents in the light of the Cavendish family manuscripts, and of the various roles played at Welbeck by Payne. It argues that the tract was written by Payne for his patrons as an attempt to apply the method of contemporary mechanics to problems of human psychology, and that it was based in part – though only in part – upon ideas about the nature of light and motion expounded by Hobbes at Welbeck during the early 1630s.