Why did Joseph Priestley support the American and French Revolutions?

Joseph Priestley’s support for the American and French Revolutions was rooted in his commitment to liberal political and religious principles, which he viewed as essential for human progress.1

Priestley argued that scientific advancement and the "perfectibility" of mankind required freedom of speech, worship, and education. His political philosophy, detailed in An Essay on the First Principles of Government (1768), emphasized that the effectiveness of a government should be evaluated solely by the welfare of the individual. As a proponent of laissez-faire economics, he sought to limit state power, a stance that aligned with the revolutionary ideals of the era.2

Because of these views, his support for the revolutions was labeled "seditious" by the English government and press, eventually leading to the destruction of his home and laboratory by a mob in 1791. His defense of these positions, including in Letters to the Right Honourable Edmund Burke, ultimately contributed to his decision to emigrate to the United States in 1794, where he found the government to be "relatively tolerable".3