An old colleague from my debate days has a post on his site that illustrates the need to keep some important distinctions at the forefront of ones mind. Libertarianism is a set of policy prescriptions; a theory about the proper scope of government. It is not a comprehensive moral theory, even though arguments in its favor may be adduced from the perspective of various different comprehensive moral doctrines. (There are utilitarian arguments for libertarian policy, Kantian arguments, existentialist ones, Christian ones, and so on.) Neither is libertarianism a psychological theory, even though, often, the arguments deployed for it draw on the pseudo-psychology (“pseudo” because everyone recognizes that the model doesn’t actually capture how we think; it’s just a useful tool) of neoclassical economics.

So libertarianism per se has nothing to say about the morality of charitable aid for more or less the same reason that it has nothing to say about why The Waste Land is a better poem than Joyce Kilmer’s Trees. Neither are questions of justice, according to a libertarian theory, even if one (or both) are more broadly moral questions. It’s worth keeping in mind that, as a theory of the basic structure, Rawls’s Justice as Fairness also has nothing to say about the desirability of an individual choice to assist the indigent or not, even though it may consider aid to the indigent in a larger structural sense of falling within the scope of its subject matter as a theory of justice.