As working conditions deteriorate, a variety of socialist philosophies emerge. Marx and Engels advocate scientific socialism, opposing both League of the Just leader Wilhelm Weitling's Christian communism and Hermann Kriege's idea of love as a unifying factor of classes in the Communist Correspondence Committee. After writing a letter criticizing those concepts, Marx and Engels are invited to join the League. In Paris, Engels learns about Pierre-Joseph Proudhon's book The Philosophy of Poverty and his ideas of mutualism and opposition to revolutionary violence...