This article discusses hearing disability in early modern Europe, focusing on medical ideas to demonstrate a profound shift in thinking about deafness over the course of the eighteenth century. Scholars have previously described changes in the social status of the deaf in the eighteenth century, pointing at clerics’ sympathy for the deaf and philosophers’ fascination with gestures as the origin of language, but there is remarkably little scholarship on the growing interest in deafness and hardness of hearing by physicians. From the seventeenth century onwards, however, medical men investigated earwax and mucus in the Eustachian Tube and developed theories about the propagation of sound waves via fluid airs and nervous juices in relation to hearing and deafness. This article argues that this focus on fluids brought about a new medical understanding of auditory perception, which viewed hearing and deafness not as dichotomous but as states along a continuous spectrum. As such, this article offers a new perspective on the study and treatment of hearing difficulties in early modern Europe, arguing that there was no solid dividing line between deafness and hearing; if anything, it was permeable and unstable.