Twain was interested in philosophy from an early age, from his introduction to the political and philosophical writings of Thomas Paine. He read widely in ancient and current philosophy, and critics have noted parallels between Twain and philosophers like Kierkegaard and Nietzsche. He allied himself to the freethinker movement, especially the writings of his contemporary Robert Ingersoll. Many of Twain’s ideas prefigure the existentialist movement of the twentieth century.