So step back, and try to imagine seeing things from an alien's anthropological perspective. The alien has all sorts of physical and psychological concepts, but no explicitly moral ideas such as 'rights'. All he does is observe what is the case; he makes no judgments about what ought to be. So when he visits Earth, what will he see?
Bob and Sally are stuck on a desert island, with a banana tree. Bob gets there first and claims it as his own -- maybe he mixes his labour with it a bit, waters and nourishes it, whatever you like. Later, Sally goes to eat a banana, and Bob stops her, pushes her back. Who aggressed against whom? From the value-neutral perspective of the alien, the answer can only possibly be that Bob was the one initiating force here.
Thus (as Hume puts it) justice, taken only in the sense that pertains to fair dealing with regard to property, is an artificial virtue, dependent wholly on human convention; it's not an arbitrary virtue, but it is an artificial one. And these conventions are revisable in light of the end they serve. As Richard puts it further down:
Anyone who truly takes liberty itself as a basic concept (rather than redefining it in terms of some other moral conception like property rights) must acknowledge that property rights can infringe on liberty. And once you make that step, the only sensible response is to assess the various institutional systems on offer, including those which render property subject to some degree of redistribution, and opt for the one that best promotes human flourishing (or whatever we think is ultimately good).
And so it would be even if some other idea than liberty is in the forefront (Richard, of course, is talking about libertarians).
