I'll begin with a standard definition of libertarianism, from Wikipedia:
Libertarianism…advocates individual rights and a limited government. Libertarians believe that individuals should be free to do anything they want, so long as they do not infringe upon what they believe to be the equal rights of others...For libertarians, there are no "positive rights" (such as to food or shelter or health care), only "negative rights" (such as to not be assaulted, abused, robbed or censored), including the right to personal property. Libertarians further believe that the only legitimate use of force, whether public or private, is to protect these rights.I must add that the creation of positive rights amounts to a violation of negative rights, because the enforcement of positive rights involves taking from some persons in order to give to others.
Whence negative rights? Negative rights arise from experience and are the distilled lessons of that experience. Experience teaches those who seek to learn from it that the preservation of personal and economic freedoms serves the general good.
In particular, as John Stuart Mill understood, personal freedoms should be preserved because through them we become more knowledgeable, more self-reliant, and more productive. Friedrich A. Hayek elaborated on Mill's insight by making the case that the personal and the economic are inseparable: We engage in economic activity to serve personal values and our personal values are reflected in our economic activity.
Moreover, as Hayek also tried to tell us, the state cannot make personal and economic decisions more effectively than individuals operating freely within an ever-evolving societal network. When the state intervenes in our lives it damages that network, to our detriment.
Thus, the general good -- the increase of our knowledge, abilities, and wealth -- is served best when the state recognizes our negative rights and acts to defend us and those rights from predators -- without and within -- and nothing more.
And that is the essence of modern libertarianism.
(This post is based on three earlier ones: here, here, and here. I should also acknowledge the foreshadowing of libertarianism in Adam Smith's Wealth of Nations:
As every individual, therefore, endeavours as much as he can both to employ his capital in the support of domestic industry, and so to direct that industry that its produce may be of the greatest value; every individual necessarily labours to render the annual revenue of the society as great as he can. He generally, indeed, neither intends to promote the public interest, nor knows how much he is promoting it. By preferring the support of domestic to that of foreign industry, he intends only his own security; and by directing that industry in such a manner as its produce may be of the greatest value, he intends only his own gain, and he is in this, as in many other cases, led by an invisible hand to promote an end which was no part of his intention. Nor is it always the worse for the society that it was no part of it. By pursuing his own interest he frequently promotes that of the society more effectually than when he really intends to promote it. I have never known much good done by those who affected to trade for the public good. It is an affectation, indeed, not very common among merchants, and very few words need be employed in dissuading them from it. [Source: Wikipedia.]Smith was describing the kind of learning from experience that is distilled in the tenets of libertarianism. Smith's "invisible hand" is really the following mechanism at work: As long as A is left alone to discover how to make a good living (without harming, stealing from, or deceiving anyone), A will discover what he is capable of producing that is most desired by B. B will be made happier for being able to buy it from A, and A will be happier because B pays him a good price for it.
The visible and heavy hand of the state can never replicate the degree of happiness that results when free markets -- operating through the invisible hand of self-interest -- integrate the ever-changing knowledge, desires, and abilities of hundreds of millions of As and Bs.)