This thesis examines diseases of the mind and brain as they were understood in early modern England, with a focus on the condition known to contemporaries as ‘frenzy’. Frenzy was an inflammation of the brain which caused dramatic changes to mood, speech, cognition, and behaviour. It was understood to be capable of spreading throughout the whole person, disordering the reason, will, memory, and passions, as well as the organs, humours, and spirits. This thesis argues that it was the disease’s wide reach within the body and the mind which made it a focal point in some of the most pressing debates of the day: debates about the nature of personhood, survival after death, free will, knowledge acquisition, religious truth, and individual accountability. It takes frenzy as an entry-point into these early modern debates, and shows how the disease variously prompted, served, and hindered the search for answers. Keeping bodies situated in the discourses which made them intelligible, and ideas situated in the contexts of their application, this thesis sits at the crossroads of the histories of philosophy, medicine, and culture. It seeks to integrate the insights of the older history of ‘madness’ into an approach informed by recent historical writing on ‘embodiment’, the ‘senses’, and ‘emotion’. As such, it makes a new contribution to an ongoing interdisciplinary conversation about the changing relations between ‘psyche’ and ‘body’ in Western culture, both in the past and in the present.