What was the phlogiston theory of combustion that Joseph Priestley supported?

The phlogiston theory, formulated in the 17th century, posited that all combustible substances contained a "fiery substance" called phlogiston. According to this theory, combustion occurred when phlogiston was released from a material, leaving behind a residue. Conversely, processes like the reduction of ores were thought to involve the absorption of phlogiston.123

Joseph Priestley was a staunch supporter of this theory throughout his career. When he discovered oxygen in 1774, he interpreted it as "dephlogisticated air," believing it was ordinary air that had been stripped of phlogiston, thereby allowing it to support combustion and life more effectively than common air.4

While Antoine Lavoisier eventually disproved the theory by demonstrating that combustion is actually a reaction with oxygen, Priestley remained unconvinced. He viewed the emerging oxygen-based chemistry as a form of dogmatic imposition and continued to defend the phlogiston framework in his later publications, such as his 1800 pamphlet, Doctrine of Phlogiston Established, and That of the Composition of Water Refuted.4