On Liberty is an essay written by John Stuart Mill published in 1859. It is split into four parts: one regarding the liberty of thought and discussions, one regarding individuality and its importance in achieving well-being, one regarding the limits of the authority of society over individuals, and the last regarding applications of the theory that came beforehand.
Mill was writing at a time when sovereigns no longer really contested their peoples' right to free speech through the press. However, he provides a lucid and logical defence of this principle which is rewarding to read. It is an evil to assert something as undeniably true, he says, because this has been asserted many times in history only for the established opinion to be proved false. So it is just as important that opinions which are generally held to be false are defended with just as much enthusiasm as they are assailed - even if they are not the whole truth, they may contain a portion of it. And because opinions and established truths maintain their vitality only when called to defend themselves (so that their proponents have to constantly re-assess and think about their so-called infallible truths) it is important that established opinion is questioned often also.
As many thinkers who we call "libertarians" are, Mill was a great believer in and defender of individuality. He believed it was a necessary component of well-being not only for each individual man, but for society as a whole. He argues for a heterogenous society in which people are not afraid to be eccentric and creative. Mill's Eurocentricity comes out in this part, as he criticizes China and Asia as a whole for their sterility. He admires the innovations of their past, but says that now they are "ruled by custom". To prevent such a downfall in Europe, men must maintain a sense of individuality, so that nations can progress rather than giving themselves into the mentality of the whole, which is ruled by custom.
In part three, Mill establishes the Principle of Liberty. It is this principle which is repeated again and again by libertarians down the ages, and today: 'No man may encroach on the freedom of action of another, except to prevent him causing harm to his fellows.' No-one has the right to impose their standards on another, or force someone to take or abstain from a certain course of action. The government only has the right to legislate the public sphere - actions taken which affect others. Civil society is clearly justified in encroaching on my liberty to stop me killing another man. Society has the right to try and discourage people from actions which it considers 'wrong' but do not harm others (masturbation, or a particular mode of worship) through social pressure, but it cannot force people to conform. Doing so would harm individuality and curse society to become trapped in the custom of today, hampering its ability to progress.
The last part of the work, entitled Applications, deals with how the principles so far established may be applied to real government. This part is of no less interest than the others, and it is indeed helpful to see Mill's principles considered in the context of real situations. He discusses things such as the institution of marriage, public education, prohibition, taxation and whether a man should be able to alienate his own freedom (by selling himself into slavery).
On Liberty is a great text for an introduction to the libertarian view of political philosophy and society. Nowadays, it is usually published along with some of Mill's other works. Mine came with Utilitarianism, Considerations on Representative Government and The Subjection of Women. Mill is easily understandable to the lay person, and he provides good food for thought to anyone from the amateur to professional philosopher, or anyone interested in public affairs.