 The scientific dimension of the Observations is in part methodical and in part substantive. As a matter of method, Hartley proposes the scientific investigation of “man.” As noted above, this is not unique; physicians had been doing just this for decades, and there were also the examples of Descartes’s Passions of the Soul (1649) and Treatise on Man (1664), and of Hume’s anonymously published Treatise of Human Nature (1739–40). (There is no evidence that Hartley knew anything about Hume, or Hume about Hartley.) What is distinctive about the Observations is that it extends Newtonian science to the study of human nature.