The Enlightenment was a major intellectual movement of the 17th and 18th Centuries which had an emphasis on the power of human reasoning to understand and improve the world, and that true knowledge ought come from experimentation and rational thought rather than unquestioned traditional thinking or religious authority. Thinkers of the day encouraged healthy skepticism toward old ideas, and believed nature operated according to a set of laws which could be discovered and understood. Despite being rooted in the Scientific Revolution, these tenets would stretch far beyond the fields of mathematics and science, as this new way of thinking promoted ideals of liberty and societal progress through education.
Phlogiston was a nebulous construct believed to exist inside materials. The theory went that when something was burned, it would release phlogiston, leaving behind dephlogisticated residue. While this was an attempt to explain observations with the tools available of the time, it gradually generated more and more questions. Antoine Lavoisier and his colleagues, through carefully planned experiments, found that something from the air combined with what was being burned, which they would identify as oxygen. These investigations also afforded Lavoisier the opportunity to establish a system of measurement, laying the groundwork for modern chemistry. When teaching these methods to others, they opted to leave out phlogiston theory so that in time, knowledge of it faded away.
As it turned out, Lavoisier had financed his research through his position as a tax collector for the French government. This made him a target of French revolutionaries who viewed him as a part of the corrupt regime. He would go on to be arrested and later executed by guillotine in 1794. His work was posthumously translated into other languages by his widow, allowing these efforts to be taken up by his contemporaries.