Karl Paul Reinhold Niebuhr (; June 21, 1892June 1, 1971) was an American theologian, ethicist, public intellectual, commentator on politics and public affairs, and professor at Union Theological Seminary for more than 30 years. The brother of another prominent theological ethicist, H. Richard Niebuhr, he is also known for authoring the Serenity Prayer, and received the Presidential Medal of Freedom in 1964. Among his most influential books are Moral Man and Immoral Society and The Nature and Destiny of Man, the second of which Modern Library ranked one of the top 20 nonfiction books of the twentieth century. Starting as a minister with working-class and labor class sympathies in the 1920s oriented to theological pacifism, he shifted to neo-orthodox realist theology in the 1930s and developed the theo-philosophical perspective known as Christian realism. He attacked utopianism as ineffectual for dealing with reality, writing in The Children of Light and the Children of Darkness (1944):

"Man's capacity for justice makes democracy possible; but man's inclination to injustice makes democracy necessary."