I argue that the imagination was a crucial concept for the understanding of marvellous phenomena, divination and magic in general. Exploring a debate on prophecy at the turn of the seventeenth century, I show that four explanatory categories were consistently evoked and I elucidate the role of the imagination in each of them. I introduce the term ‘floating concept’ to conceptualise the different ways in which the imagination and the related ‘animal spirits’ were understood in diverse discourses. My argument is (...) underpinned with a general discussion of the imagination, after which I focus on a unorthodox tradition which attributed special powers to this internal faculty. Many physicians had explained psychosomatic phenomena by referring to the influence of the imagination on the human body; but some philosophers held that the imagination could also exert power over external objects. During the renaissance these theories acquired negative associations and became linked with illicit magic. Furthermore, I show that the relation between both animal spirits and imagination, and the power of the latter over external bodies, points to the importance of a ‘history of vapours’; and this suggests an adjustment to Keith Hutchison’s argument on occult qualities in mechanical philosophy. The accompanying paper gives more evidence of these claims and elaborates on the relation between the natural and the moral. (shrink)