2009 reprint of the 1869 [Fourth Edition]. Mill's On Liberty addresses the nature and limits of the power that can be legitimately exercised by society over the individual. One argument that Mill develops further than any previous philosopher is the harm principle. The harm principle holds that each individual has the right to act as he wants, so long as these actions do not harm others. If the action is self-regarding, that is, if it only directly affects the person undertaking the action, then society has no right to intervene, even if it feels the actor is harming himself. On Liberty involves an impassioned defense of free speech. Mill argues that free discourse is a necessary condition for intellectual and social progress. We can never be sure, he contends, that a silenced opinion does not contain some element of the truth. He also argues that allowing people to air false opinions is productive for two reasons. First, individuals are more likely to abandon erroneous beliefs if they are engaged in an open exchange of ideas. Second, by forcing other individuals to re-examine and re-affirm their beliefs in the process of debate, these beliefs are kept from declining into mere dogma. It is not enough for Mill that one simply has an unexamined belief that happens to be true; one must understand why the belief in question is the true one. Mills's On Liberty remains a classic text in political theory.