human body and chemistry he was able to develop a medicine for physical injuries. Boerhaave attributed fevers to the body's response to a stressful situation or shock, similar to the way that chemical reactions produce heat, in which the body encountered an unexpected onset of heat or freezing temperatures. A German-born physician, Franciscus Sylvius (1614–1672), is best known in 18th-century European medicine for his contributions to the understanding of the biochemistry of the body and the tubercles, and as one of the co-founders of an iatrochemical school. In continuation of humoral medicine, Sylvius did deem that diseases resulted from excesses of the humors in the body, but he saw it as a more chemically driven excess, specifically one of too much acid or alkali solution in the body. Sylvius had his own laboratory in which he ran experiments on acids and alkali solutions to see the result when different mixtures were made. Much of his theories of the human body were based on the digestive processes. His understanding was that digestion helped food undergo a fermentation reaction. He rationalized that the body functioned mainly as a result of chemical reactions, of which acids and alkali were the essential reactants and were products which needed to be kept in balance to be in a healthy state. Although Sylvius did not take on the more observation-based style of medicine that was being so championed in the 17th and 18th centuries, his emphasis on the chemical reactions and knowledge helped support this more observation-driven scientific approach to medicine. It is known that many of Sylvius' inquiries did help in the future discoveries of certain enzymes driving food digestion and bodily reactions. The understanding of iatrochemists helped to drive new knowledge of how drugs work and treat medical conditions. Specifically, one English iatrochemist, Thomas Willis (1621–1675), considered the effect of diaphoretics (sweat-promoting drugs) as resulting from the mechanisms of the drug entering the blood and associating or disturbing blood and flow which produces a state of heat and sweat. He also hypothesized that the working of opiates came from an interaction with a salt in the body that created a painless and woozy feeling when it reached the brain. In his treatise De fermentalione (1659), Willis rejected the four Aristotelian elements of earth, air, fire and water, stating that they provided no special insight into "the more secret recesses of nature." Willis settled on