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, Harris, Gibbs, and Branch, I detail the role of bodily fluids and nervous spirits in “conveying the mischief” by which imagination tends to ruffle our calm. Minds are often surprised by their own habits, and various forms of regimen were recommended in these works of medical psychology and moral physiology to ‘pinion’ the imagination and still the roving thoughts. I anchor these local discussions within a broader enquiry into mind-wandering and ‘stimulus-independent thought’, and sketch a rich neurophilosophical background to Hume’s views on the bodily bases of custom and habit.