Centred on the eclectic sources of early modern neo-atomistic medicine, this chapter examines the physiological theory of German alchemist Daniel Sennert (1572–1637) and Dutch engineer Isaac Beeckman (1588–1637). Both university-trained physicians, they followed Galenic medicine in explaining the structure and functioning of the human body at the level of its smallest components. However, they interpreted the traditional theory of elements and mixture into atomistic terms by postulating the discrete structure of matter into particles, minima, and atoms. In exploring Sennert’s Institutiones medicinae (1620), De chymicorum … liber (1629), and Beeckman’s notebook (ca. 1616–1620), I consider their explanations of elements and mixture in relation to the physiological notion of temperament. In doing so, I look at the medical sources that Sennert and Beeckman used in support of their atomistic claims, from Galen and Avicenna to late Renaissance physicians such as Jean Fernel. This leads me to explore the common and distinct features of their interpretations in comparison with Santorio Santori’s theory of elements and mixture in Methodus vitandorum (1603).