Although he obtained a medical degree at the University of Caen in 1618, Isaac Beeckman never practised medicine. Instead, he developed an atomistic conception of Galenic physiology by discussing, throughout his notebook, the constituents and functioning of the living body. Interestingly, Beeckman applied his atomistic interpretation to the notion of temperament as the balanced proportion of elemental qualities, which defined the state of health. In this chapter, it is shown how his atomistic views on health and temperament amalgamated the Galenic theory of elements, mixture, and digestion. In appraising related interpretations of the body by late Renaissance novatores, Beeckman proposed an original theory of the organism, which put forward a mechanistic conception of metabolism as characterized by the rarefaction and condensation of atomic matter.