Why did Joseph Priestley call oxygen dephlogisticated air?

Joseph Priestley called oxygen "dephlogisticated air" because he interpreted his discovery through the lens of the phlogiston theory, a prevailing scientific framework of his time.12

According to this theory, combustible substances contained a hypothetical principle of fire called phlogiston, which was released during combustion. Priestley believed that ordinary air supported combustion and life only because it had the capacity to absorb phlogiston. When air became saturated with phlogiston, it could no longer support these processes.12

When Priestley isolated the new gas in 1774—by heating red mercuric oxide—he observed that a candle burned more brightly and a mouse thrived in it. He concluded that this gas was "dephlogisticated" because it was essentially air that had been stripped of phlogiston, or was entirely free of it, thereby possessing a superior capacity to absorb phlogiston from burning objects or respiring animals. Despite the later success of Antoine Lavoisier’s oxygen theory, Priestley remained committed to the phlogiston framework throughout his life.21