One of the most significant works of political philosophy, John Stuart Mill's On Liberty (1859) defines and defends individual liberty, a cornerstone of classical liberal thinking. Defining liberty as freedom from interference by state power or popular moral opinion, Mill justifies the individual's right to this liberty by focusing on the role self-development plays in human well being. He argues that society should protect the development of individuality to aid both social progress and innovation. These principles—outlined by Mill over 150 years ago—still influence political debate today.