Reinhold Niebuhr (1892-1971) was an American theologian and political philosopher known for his influential ideas on ethics and politics. He is best known for his concept of Christian realism, which emphasizes the complexities of human nature and the moral dilemmas faced in political life. Niebuhr's writings often addressed the tension between idealism and realism, advocating for a pragmatic approach to social justice. Niebuhr's most famous work, The Serenity Prayer, reflects his belief in the need for wisdom and courage in the face of life's challenges. He also contributed to discussions on social ethics and the role of religion in public life, influencing both religious and secular thought in the 20th century.