How did Joseph Priestley connect freedom of speech with scientific progress?

Joseph Priestley, an influential English clergyman and scientist, argued that scientific progress and the broader goal of human perfectibility were inextricably linked to the protection of individual liberties. In his 1768 work, An Essay on the First Principles of Government, Priestley posited that freedom of speech, along with freedom of worship and education, was a fundamental requirement for the advancement of knowledge and society.1

Priestley believed that progress relied on an environment where ideas could be freely exchanged and tested, rather than suppressed by dogmatic authority. This conviction extended into his scientific practice; he viewed the imposition of rigid scientific theories—such as those proposed by Antoine Lavoisier—as analogous to the restrictive religious and political dogmas of the Anglican establishment. For Priestley, true scientific inquiry required a humble, experimental approach that encouraged the observation of nature without the interference of speculative, forced consensus. By advocating for a society that prioritized the welfare of the individual, he sought to create a framework where both scientific discovery and human happiness could flourish through open discourse.21