Toward a Fallibilist Synthesis
I see Trent's post has sparked another round of the long-running natural-rights-vs-consequentialism debate. As some here already know, I think that either approach by itself is incomplete, and I want to elaborate a bit on why. I think there is a distinct third approach to libertarianism-- the fallibilist approach-- which naturally implies a need for both natural-rights and consequentialist reasoning. I haven't always considered myself a fallibilist libertarian, but I'm pretty sure that's what I am now.
The fallibilist premise is simple: human beings are imperfect and are in particular too imperfect to judge well, individually or collectively, how to order other people's lives. You might call it "militant humility" instead, or the "no-angels-in-the-form-of-kings" thesis to tip the hat to Jefferson. It's clear, I think, how this leads to libertarian political beliefs. (By the way, as a nice side effect, fallibilism implies that the recent spate of results in behavioral psychology about systematic irrationality in decision-making strengthen, rather than weakening, the case for libertarianism. If we are systematically flawed in our efforts to see what is good for ourselves, how much worse must we be at seeing what is good for others, about whom we can know far less and with whose interests we can have at best partial empathy!)
Fallibilism says that the rights problem-- what are our ethical obligations to one another in a good society?-- and the consequences problem-- of possible measures we could take, which one will truly end up doing the most good in the long term?-- are both intrinsically difficult, and in reasoning about either one we are likely to make errors. Moreover, subtle errors followed logically through long chains of reasoning can easily lead to absurd conclusions. We therefore need consequentialist reasoning to help correct the errors that inevitably crop up in thinking about rights, and natural-rights reasoning to correct the errors that inevitably crop up in thinking about consequences.
The differences between the two approaches make them useful for checking and balancing each other. When your natural-rights logic leads you to a conclusion which seems clearly to have horrible empirical consequences, it's a good sign that you may have made a subtle error somewhere in your assumptions about what rights people really have. And when your consequential reasoning leads to what seems a clear violation of natural rights, there's probably some long-term or indirect consequence you've forgotten to consider. If you only use one or the other, you're almost certain to go astray. There is an analogy here to independence of developers and testers in software engineering, but I'll spare you the geeking.
Now as to Trent's question, I'm not sure where fallibilism leads one to be on the old left-right political spectrum. I have swung, myself, from left to right and (sort of) back, and may well continue to do so. The one observable pattern so far is that I tend to find myself naturally allied with whichever side is out of power; the ones in power always seem more arrogant and corrupt, those out of power the lesser evil. Fallibilism could readily explain this-- it's those in power who are actually telling other people what to do, not just thinking about how they'd like to do so-- but I doubt it's necessary. In consequentialist form this is just the "gridlock strategy," and Murray Rothbard, as extreme a natural-rights guy as you'll find, swung back and forth in a similar way as well.