On the currently vogue topic of credit card companies targeting the mentally not-all-that-with-it in order to turn a turn a greedy buck -- and how this is somehow damning of libertarianism, my colleague Jason Kuznicki makes a good point:
I am always amazed at how border cases are dragged out, again and again, as if they proved something against libertarianism. Border cases — How old before you can vote? How demented before a contract doesn't bind? — are a problem in all political systems, because all systems start with a presumed community of citizens and/or subjects. We always have to draw boundaries between the in-group and the outliers before we have a polity in the first place.
What makes the classical liberal/libertarian approach so valuable is in fact that it draws so few boundaries. Where other systems depend on class boundaries, race boundaries, religious boundaries, and so forth — with annoying boundary issues at every stop along the way — libertarians make it as simple as I think it can be. We presume that all mentally competent adults are worthy of liberty until they prove themselves otherwise.