Herman Boerhaave was not the first physician to study the bodily fluids by chemical methods. This chapter discusses his precursor, in particular Paracelsus, Van Helmont, and Sylvius. In a time dominated by humorist, anatomical, and mechanical views of the body, these men belonged to a new school of iatrochemistry—or medical chymistry—that relied on chemical methods to understand the human body. Focusing on the case of saliva and digestion, this chapter examines how Boerhaave rejected medical chymistry to solidify his own chemistry of medicine. Although Boerhaave was less dismissive of chymistry than he publicly led on, ultimately he was able to develop an irenic and inclusive model of physiology, establishing it as the standard for decades to come.