Liberty -- simply defined -- is the ability to do as one wishes, as long as one does not harm others. It follows that the members of a society can enjoy liberty only to the extent that each of them refrains from doing harm to the others. The exercise of such mutual restraint requires that each member of society must
1. agree to the same definition of harm; and
2. trust that other persons will not harm him, so that (a) he is not tempted to act pre-emptively against them or (b) he is not quick to decide that he has been harmed when there is ambiguity about the intent of effect of another's actions; and
3. given 1 and 2, be unwilling to do harm to other persons, out of (a) respect or affection for them, (b) a desire for respect or affection from them, or (c) empathy for them; or
Alternatively, given condition 1 (an agreed definition of harm), each member of society must
4. be deterred from causing harm by the likelihood of being punished.
Conditions 1, 2, and 3 are the pillars of civil society -- a society that regulates itself and operates without interference by a state. The fragility of civil society can be seen in the fragility of the conditions for its existence. There is no room in those conditions for
5. disagreement about harm, except to the extent that members of society are willing to compromise their views in return for something of value from others (e.g., respect, affection, mutual defense).
6. persons who are untrusting, untrustworthy, sociopathic, or unempathic.
But, given the likelihood of conditions 5 and 6, it is futile to expect civil society to arise absent condition 4. And condition 4 cannot arise as an agreement among all members of society as long as conditions 5 and 6 persist. (Anarchists and anarcho-capitalists seem blind to this contradition, which is inherent in their philosophies.)
Society therefore quickly devolves into a "warre of every man against every man," or at least a war of faction against faction. Or it does so unless enough "men" band together to impose upon all a uniform definition of harms and a state that is capable of enforcing that definition. "Enough" may be a majority or even a super-majority, but it is unlikely ever to be a consensus. Human nature is too varied for that.
The best one may hope for, then, is a state that is (a) accountable to all members of society (even those who do not endorse its ruling faction or its existence) and (b) liberal (in the old, anti-statist meaning of that word). Minarchism prescribes such a state. It follows, therefore, that minarchism is the only viable libertarian philosophy.
Defense, Anarcho-Capitalist Style
But Wouldn't Warlords Take Over?
My View of Warlordism, Seconded
QandO Saved Me the Trouble
The Meaning of Liberty
The Harm Principle
Footnotes to "The Harm Principle"
The Harm Principle, Again
Actionable Harm and the Role of the State
Rights and "Cosmic Justice"
A Flawed Defense of Anarcho-Capitalism
Mises on Liberty and the State
Varieties of Libertarianism