Although Europeans knew Priestley best as a scientist (he had discovered oxygen gas in 1774), Americans knew him best as a defender of religious freedom and as an advocate for American independence. Immediately upon his arrival, he was by various political factions vying to gain his support. Priestley declined their entreaties, however, hoping to avoid the political discord that had embroiled him in Britain. He wrote to John Adams that he "made it a rule to take no part whatever in the politics of a country in which I am a stranger, and in which I only wish to live undisturbed". (Priestley never became a citizen of the United States.) He also turned down an opportunity to teach chemistry at the University of Pennsylvania at this time.