Some claim that NAP (the non-aggression principle) cannot be used as a grounding for the libertarian ethic, since it begs the question in favor of libertarianism by defining aggression in accordance with libertarian preconceptions.

This is incorrect. NAP presupposes the notion of aggression that is not libertarian, but commonsensical. Since definitions of everyday concepts can be debated endlessly, the most intellectually respectable thing to do in this context is to follow the Wittgensteinian dictum “meaning is use” and accept their most mundane, everyday definitions (provided that they are sufficiently unambiguous) rather than, say, sophistically conflate them with definitions of related concepts. Isaiah Berlin understood this well when he wrote: “Everything is what it is: liberty is liberty, not equality or fairness or justice or culture, or human happiness or a quiet conscience”.

“Aggression” belongs to the category of such mundane concepts – it is not a piece of abstruse philosophical jargon, but a basic term readily understable to laypeople as the initiation of physical force against persons and property. Whenever one wishes to detach the concept of aggression from its inherently physical connotations, one has to qualify it with an adjective, as in “verbal” or “psychological aggression”. In this sense it is similar to the concept of justice, which is typically intuitively understood as “equality before the law”, but which, as Hayek understood all too well, loses all clarity and comprehensibility when qualified with the adjective “social”.

Having understood the above, it is then not difficult to appreciate the common-sense appeal of libertarian arguments grounded in NAP, regardless of whether one agrees with their finer philosophical details. In other words, regardless of whether one agrees that aggression is wrong because it is a violation of human nature (whose defining characteristic is the ability to act freely, an ability that cannot be exercised when hampered by physical force), or because it necessarily entangles the aggressor in a performative contradiction, it is then not difficult to appreciate the moral plausibility of the presumption against aggression, and, by the same token, the moral plausibility of the presumption for libertarian freedom.