The phlogiston theory was a scientific hypothesis developed in the 17th century to explain combustion and oxidation. It proposed that a substance called phlogiston was released during burning, making materials lose weight. According to this theory, materials rich in phlogiston would burn easily, while those with little would not. This theory was widely accepted until the late 18th century when Antoine Lavoisier conducted experiments that led to the discovery of oxygen. Lavoisier's work demonstrated that combustion involves a chemical reaction with oxygen, ultimately discrediting the phlogiston theory and paving the way for modern chemistry.