Bleeding heart libertarianism is a term that, to some, may sound like an oxymoron. However, according to the members of the blog BHL (Bleeding Heart Libertarians), this is not at all the case. They argue that it is, in fact, not only possible, but unavoidable that, if one cares about the poor, or other "marginalized" groups, one should be a libertarian. This is where bleeding heart libertarians essentially agree with "conventional" libertarians--that liberty brings prosperity and justice. However, where bleeding heart libertarians depart from the conventional libertarian narrative is in their view of the value of liberty relative to other moral goals people may have (i.e., helping marginalized groups, like the poor).
Bleeding heart libertarians argue that most people consider liberty (as defined by "conventional", say, Rothbardian libertarians) to be only one of the desirable human moral goals. This implies that, if faced with a situation in which one would have to choose between liberty and all other moral ends, most people would be willing to trade off some of their desire for freedom for the sake of keeping or increasing the attained level of other moral ends.
The next step bleeding heart libertarians make is to say that they, like most other people, don't always place freedom at the top of their moral value scale. So, when it comes to valuing freedom relative to other human moral ends, bleeding heart libertarians don't want to change people's preferences. they take people's preferences as given. The twist they introduce is the argument that, even for an average Joe, who is willing to trade off some of his desire for freedom for the sake of attaining other human moral ends (like helping the poor), the best possible world is the one in which liberty is maximized. Why?
The reason why even an average Joe would, if he understood economics properly, choose a world in which freedom is maximized is that the technical relationship between attaining freedom and attaining other important human ends is such that there is no trade off between freedom and the other moral ends average Joe cares about. According to this view, more freedom leads to a better attainment of the other moral ends average Joe cares about.
This view has some important implications. First, to persuade people that libertarianism is the way to go, one does not need to change people's preferences for freedom. Instead, one should try to explain to people why more freedom leads to a better attainment of the other moral ends people hold as important. Thus, the emphasis here is on using economic science to determine whether the claim about the technical relationship between freedom and the attainment of other human ends is true.
Is it really true that more freedom leads to a better attainment of other important human ends, including those moral ones, like helping the poor? From what I have learned about the functioning of economic systems so far, I would venture to say that this claim is generally true in most situations. However, claiming that more freedom leads to a better attainment of other important human ends in all situations would be, at best a stretch, and a vacuous generalization at worst.
The other implication is that, if it turns out that bleeding heart libertarians, "conventional" libertarians, and I were wrong about the technical relationship between freedom and the attainment of other important human ends, then an average Joe would not prefer a world in which freedom is maximized, and bleeding heart libertarians would not advocate such a world. They would advocate a world in which some level of freedom is attained, but not the maximum level of freedom. If, in fact, the attainment of more freedom was possible only at the expense of not achieving some of the other important human ends, then an average Joe would want to give up some of his desire for freedom for the sake of attaining other important human ends.
This, in my view, is the crux of the disagreement between bleeding heart libertarians and "conventional" libertarians. "Conventional" libertarians argue that liberty should be the highest human value even if that meant giving up some of other important human ends. Thus, "conventional" libertarians are making an argument that human preferences for freedom should have a specific form--a form in which one is not willing to give up any of his desire for freedom even when this is for the sake of achieving some other human ends. On the other hand, bleeding heart libertarians are arguing that, regardless of our preferences for freedom, once we understood the relationship between the attainment of freedom and other human ends, we would choose a world in which freedom is maximized.
Since bleeding heart libertarians take human preferences as given, and "conventional" libertarians argue that human preferences for freedom should take a specific form, there is no common ground for debate between "conventional" libertarians and bleeding heart libertarians. While they agree on the nature of the technical relationship between the attainment of freedom and other important human ends, their disagreement on the proper shape of human preferences for freedom remains, and it will remain until one or the other group modifies its preferences for human moral ends.