Dictionaries give us two definitions of “legitimacy”: “the quality of being legal” and “the quality of being reasonable and acceptable”. The two meanings are intertwined: we expect reasonability from the laws and we infer the content of a law we do not properly know from what we regard as reasonable. Unreasonable laws are not acceptable to the people and Cesare Beccaria warned us about how unreasonable prohibitions engender more and new crimes.
Political Realism and Legal Positivism cross their paths when it is time to discuss what is the ultimate foundation of obligation, both political and legal: facts and force. An overwhelming force deployed upon individuals and peoples will always be able to impose what is reasonable and acceptable. For Thomas Hobbes, as fear is not a sufficient reason for annulment of covenants, the feeling of terror from the subject to the sovereign does not challenge the legitimacy of his power.
Libertarianism is, in principle, a political stance on the state that denies its legitimacy or, at least, denies unlimited sovereignty. And we stress “in principle” because we want to point out that not all versions of Libertarianism accomplish the said aims. In this regard, we want to single out one crucial trait of every Libertarian political theory: is it possible a stateless order of cooperative coordination between individual plans? Does its existence depend upon our own volition and agreement? We want to make a distinction between two strains of Libertarianism: the one which affirms the possibility of a stateless society and the one which does not.
Paradoxically, the affirmation of the possibility of a stateless order of cooperation legitimates the Hobbesian stance on unlimited sovereignty: having at their disposal the alternative of a stateless political order, individuals opt freely for a Leviathan. What we have to decide now is the extension of the power of the government, but at this point there is no restriction left to the power of the Leviathan to determine its own limits.
On the other hand, for the strain of Libertarianism that regards the absence of a state as impossible or not desirable because, for example, the justice is an artificial virtue that demands a government to enforce it, the state is a fact that has no reasonable alternatives. As the theft that compels us to choose between our bag or our life, there are no reasonable options left to us but accepting the power of the state. As David Hume pointed out, tacit conventions as the stability of the possessions require a political order to enforce it. Therefore, the factual power of the state will be legit only as long as it enforces the tacit order of human cooperation that allows individuals to fulfil their plans. Notwithstanding this last strain of Libertarianism does not deny the legitimacy of the state, it does consistently deny the legitimacy of any type of unlimited sovereignty.