 Theories of the emotions played a role – often a pivotal one – in the important early modern debates about causation and the proper forms of explanation. As part of their embrace of the new science, many seventeenth century philosophers considered the emotions to be susceptible, at least in part, to mechanical explanation. Although Descartes offered a teleological defense of our propensities to experience emotions, his account of their physiological underpinnings is mechanistic. Malebranche too considered the functions of the emotions and the way in which that functioning has been corrupted, but emphasized that the emotions are communicated through strictly mechanical operations. Hobbes and Spinoza went yet further, rejecting any talk of final causation in order to treat the behavior of the emotions as completely continuous with bodily movements, and indeed reducing the appearance of goal-driven behavior to the motions of the passions. In contrast, Shaftesbury criticized Locke and Descartes for failing to appreciate the natural teleology of our emotional constitution, and dismissed all physiological accounts as beside the point. Many other British philosophers showed less interest in the metaphysics of explanation and more in defending an empiricist account of the origins of our ideas. But the rejection of innate ideas often drove them to focus explanations of the emotions on the hydraulics by which pains and pleasures push our ideas. This naturalistic approach was particularly marked in eighteenth-century associationist psychology, often hand in hand with ‘Newtonian’ ambitions to produce a “science of man:” examples include Hume’s Treatise of Human Nature (1739–40), and such lesser-known works as David Hartley’s Observations on Man, his Frame, his Duty, and his Expectations (1749) and an anonymous tract of 1741, An Enquiry into the Origin of Human Appetites and Affections, Showing How Each Arises from Association: for the use of young gentlemen at the universities (cited in Gardiner 1970, 221), as well as in the works of Condillac.