How and why should physicians translate qualitative impressions into quantitative values? This chapter analyzes some answers to that question in early modern British mathematical medicine. Focusing on the geometrical medicine of Archibald Pitcairne and the British followers of the mathematical experimentalism of Santorio Santori (Sanctorius), I ask how mathematical physicians attempted to quantify the medical concept of temperament or complexion by adopting the method of Newtonian mathematical physics and re-trying Sanctorius’s static weighing experiments. I argue that, insofar as Pitcairne’s program called for the imitation of Newtonian astronomers, it had difficulty translating temperament into a useful and meaningful quantity. The restaging of Sanctorius’s experiments, however, gave mathematical physicians practical opportunities to quantify temperament through the externalization and systematic recording of body weight, food intake, and bodily excreta.