Thomas Jefferson Thomas Jefferson is a American leader and political philosopher, author of the Declaration of Independence, and the third president of the United States. (1801-1809) Jefferson was among the most brilliant American exponents of the Enlightenment, the movement of 18th-century thought that emphasized the possibilities of human reason. A Virginia aristocrat, he had the time and resources to educate himself in history, literature, law, architecture, science, and philosophy; as diplomat and friend of French and British intellectuals ....