Tempering Occult Qualities: Magnetism and Complexio in Early Modern Medical Thought

In medieval natural philosophy and medicine, magnetic attraction was the most commonly invoked example for the effects of so-called ‘occult qualities’ or ‘occult powers.’ According to this conception – which dates back to Galen, Alexander of Aphrodisias, and Avicenna – magnetism was caused by an insensible quality and not, therefore, by one of the four primary qualities (hot, cold, wet, dry). Already disputed in medieval times, however, was whether the magnet’s ‘temperament’, ‘mixture’ or ‘complexion’ might not account for the attraction of iron. In the early modern period, trained physicians above all increasingly refuted ‘occult qualities’ in magnetism, while at the same time retaining a Galenic framework. They argued instead for more elaborate theories invoking the magnet’s and iron’s ‘complexion’ or their single primary qualities, such as ‘humidity’ or ‘heat.’ Medical concepts were often combined with meteorological ideas for causal theories of natural phenomena like magnetism. By telling this unheard story of ‘complexion’ in theories of magnetism, we show not only how medical theories were transferred from medicine into other fields of research, but also that an established narrative in modern historiography is highly questionable: contrary to what was assumed by the contemporary critics (e.g., Descartes) and many modern historians, several Galenic physicians did not subscribe to a theory of occult qualities (in the case of magnetism) in the sixteenth and seventeenth centuries.