How can you not see the difference between a lease, which was signed by the tenant, and a decree that was forced upon a citizen by the majority? One is a voluntary agreement, the other is force.The commenter makes a number of rather obvious errors, most notably that that Libertarian argument does not usually consist of how to justify the initiation of coercion, but rather that the initiation of coercion is intrinsically unjustifiable. Of course, as I discuss in the post, the Libertarian argument is fundamentally in bad faith: agreements they don't like are unjustifiable because they entail the initiation of coercion; agreements they like aren't the initiation of coercion because they are justified. I find this "argument" as tedious and stupid as Divine Command Theory.
But this comment, while completely stupid, does suggest a more interesting question: what is an agreement? What do agreements mean? When is an agreement voluntary? The Libertarian position is straightforward: Agreements are what Libertarians want them to be, they mean what Libertarians want them to mean, and they're voluntary when Libertarians like them (and involuntary when they don't like them). Libertarians interpret "voluntary agreement" like Christians interpret the Bible.
But there are more subtle issues (more subtle than a Libertarian is like shorter than Manute Bol) about agreements.
What is a voluntary agreement and what does it mean? Well, first of all, an agreement has to provide mutual benefit: all parties to an agreement have to be better off overall than if the agreement did not happen. Second, all parties to a voluntary agreement must be able to forego the agreement without harm. Finally, an agreement cannot be fully explicit: there must be a social process to interpret the meanings of agreements. But even with this definition, there are still notable controversies.
First, although an agreement has to provide mutual benefit, there's no determinist how that mutual benefit is allocated among the parties. The allocation of the "surplus benefit" is a matter of bargaining power, not logic, which then raises the question: what are legitimate and illegitimate ways of obtaining and exercising bargaining power? Is possession of money legitimate bargaining power? How about votes? How about violence, threatened or actual? People have any number of positions on these issues, but there's no single rationally correct answer; they're political questions.
Second, what constitutes "harm"? It's relatively uncontroversial that getting killed or beaten constitutes harm, so that giving my wallet to a mugger who points a gun at me is not a voluntary agreement. But what about natural harms? Does freezing or starving to death constitute a harm that renders an agreement involuntary? How about becoming stateless, ejected from the protection of a society? Is it harm to lose some portion of the mutual benefit of an agreement because of a change in bargaining power. No one is self-sufficient; we all need to rely on others for our physical survival. Does our mutual dependence entail at least some obligation on the part of others to assist the survival of others? Again, these are not rational but political questions.
Third, how do we resolve disagreements between parties to an agreement about what a particular agreement actually means? What, precisely, have the parties agreed to? What do we do about these disputes? Again, how to interpret an agreement is as much a matter of not rationality but politics.
It's not enough to lay out just a consistent set of answers to these questions, because there are a very large number (perhaps infinite) number of consistent answer sets. Why should I prefer your set of answers to another?
One problem (out of many) with not only Libertarianism (right-anarchists) but also left-anarchism is that anarchists simply push all the interesting political questions to "voluntary agreements" and assume all these problematic questions are thereby solved at a stroke. I think this position is at best naive, and at worst dishonest.