This article examines Thomas Morgan’s medical ideas in the context of Newtonian iatromechanism in late seventeenth and early eighteenth-century Britain. In existing historiography, Morgan has been studied as a deist, but his medical writings have received little attention. In fact, Morgan has been classified as a Newtonian iatromechanist, but viewed as a minor figure in comparison with other Newtonians such as Archibald Pitcairne, George Cheyne, and James Keill. Such an assessment is partly due to the fact that Morgan did not have a social connection with Isaac Newton while others did, but also because Morgan’s medical writing has been considered second-class and largely repeating earlier Newtonian works. However, close examination shows that though Morgan shared some common grounds with his predecessors, he developed original interpretations in many areas. Just like Pitcairne and his followers, Morgan looked upon an animal body as a complicated living machine whose life lay in the proper circulation of blood and other fluids through the small pipes distributed throughout the body. However, Morgan differed in that he, more than others, saw heat as an important factor in physiology. He thought that fire was condensed light and that the subtle substance of elementary fire was diffused through the universe. Heated air, or the air containing elementary fire breathed in acted as an expansive force to counterbalance the attractive forces between small particles in the animal body. According to Morgan, fever, which was considered a distinct disease at the time, was caused when the heated air in the blood was not properly evacuated or when the warmed body fluids were not cooled down. Morgan also argued that the main agent of digestion was the heat operating in the stomach. His heat-based interpretation was novel but fitted into the existing understanding of physical matters including the animal body, and it provided explanation about some physiological phenomena which were not fully explained before. The explanatory power of Morgan’s medical writing earned him a good reputation as a physician among his contemporaries. In the context of the transition from iatromechanism to vitalism, part of Morgan’s heat-based physiology may be considered as contributing to the shift, by proposing that the capacity to maintain body heat - that is, the ability to absorb, retain, and emit elementary fire and air - belonged solely to the living body.