"In ancient Egypt it was thought that only some sort of supernatural energy source could power the living. The Roman physician Galen claimed that 'innate heat' and 'spiritedness' were the 'vital forces' created by the 'pneuma' that was extracted from inhaled air in the lungs. The vitalists claimed that living organisms were imbued with a mystical force that existed on a higher plane that transcended the realm of physical law. It was a theory that was in harmony with, and perhaps bolstered by religious doctrine. 'Nothing is so firmly believed as that which we least know' wrote French philosopher Michel de Montaigue. To Lavoisier, vitalism was the same as the phlogistion theory of combustion-a theory born from ignorance, a theory that conveniently could not be measured, a theory one must take on faith.

Lavoisier suspected that the vital spark generated by respiration was the same as the combustion of inanimate material like fuel in a fire. His experiments established a porwerful corollary: animals inhale oxygen, the active compontent of air responsible for combustion and exhale the same gas that is released from combustion-an inert gas called "fixed air" later identified as carbon dioxide. 

He created a fancy experiment where he measured the output of heat from a guinea pig and compared it to combustion of carbon. "Respiratory gas exchange is a combustion, like that of a candle burning." Life was a slow combustion, proving the vitalists wrong. 

With Lavoisier's experiment the field of energy metabolism--long muddled by the ambiguous concept of vitalism--leapt f orward as it suddenly found clarity. Criticially, his experiments illuminated the beginning and end of the metabolic map: food + oxygen = energy + carbon dioxide.