Associated ideas, complained Locke, follow one another “without any care or attention.” In a brilliant inversion of Locke’s nervous worries about the perils of misassociation, Hume resolved the sceptical despair brought on by philosophical reasoning only by returning to mindlessness: “carelessness and in-attention alone can afford us any remedy. For this reason I rely entirely on them” (Treatise, I.4.2). How did British natural and moral philosophers in the early eighteenth century think about what happens when the mind is elsewhere? How did they theorize the processes by which thoughts, fancies, memories, daydreams, and feelings come to mind without prompting either by reason or reality, by the will or by the world? Examining works by Mead...