In this article, Herman Belz examines the political career of one of America’s greatest leaders: Abraham Lincoln. Situating Lincoln within the natural law tradition, Belz argues that Lincoln’s deliberations about the contentious and sectional conflict over slavery as well as his understanding of the correct exercise of his executive power as President were informed by the natural law. Lincoln’s arguments against slavery, immortalized in the famous Lincoln-Douglas debates, were grounded on both natural-law and natural-right principles. He asserted classical natural-law notions of the nature of the human person and the teleological principle of political or legislative prudence as striving toward the good of the individual and the common good of society. Concomitantly, Lincoln drew on the modern natural-rights theories that were foundational to the American political experiment and constitution through such doctrines as the right to labor and the goods produced by labor and the right to self-government. With a combination of sarcastic humor and careful philosophy, Lincoln affirmed, clarified, and vindicated the unique amalgam of classical natural law and modern natural rights that defined American republicanism. In-so-doing, Lincoln not only brought about the emancipation of the slave population of America, but also preserved the integrity of the Union and the imperative for earnest public deliberation on issues of great moral import.