[N]o specific set of rules is the presumptively legitimate baseline from which we have to start. Property is a social construct, and when we construct it, all the different sets of rules that would define different systems of property are logically on a par.
One might object that this account gives too much weight (or "metaphysical force") to government or social institutions. Hilzoy responds with an analogy to sporting and other conventions: society "constructs" property like it does the rules of baseball, creating the possibilities of "theft" and "striking out", respectively. No great "metaphysical force" is required for either act of creation.
I think more needs to be said here. The rules of baseball are rather arbitrary, after all. They are merely conventional. Yet people typically think that there's more to property rights than that. Thieves aren't just breaking the law, they're also doing something wrong. Rights ought to be respected; they constitute moral requirements. So it seems that if institutions create rights, then they create moral requirements. That would be a distinctively significant form of "metaphysical force".
A related concern would be that the account gives the institutors too much discretion or normative power. If there are a wide range of possible "rights" that could be instituted, which society is to decide between, this seems to imply that society gets to decide what's morally required -- and that would be absurdly relativistic. (N.B. Hilzoy, to her credit, pre-empts this obvious objection by declaring: "I don't mean that we can make [property] any old way we want without criticism." But I learned the hard way that some critics simply ignore such caveats if they're not fully explained, so it's probably worth making clear exactly how the relativistic conclusion is to be avoided.)
I would answer the problem like this: society can (in the morally neutral sense of "has the ability to") institute a wide range of "property" frameworks and associated legal rules. But it shouldn't be assumed that the rules have moral or normative force. We don't get to decide which frameworks are good ones. (Morality is no "social construct" in this sense.) The normative status of internal violations is derivative upon the normative status of the institution itself. I explain all this further in my post on 'Institutional Rights'. The core thing to note is that we can escape the above objections because institutional rights are not normatively fundmental, and this means that they are not creating moral requirements out of thin air. The state has neither the rightful discretion nor the "metaphysical force" to perform such a feat.
It won't do to have just any old system of property. We want to have rightful property: property such that its theft would be morally wrong. My claim, in short, is that society constructs only the "property" component of "rightful property". The normative component has an independent foundation (i.e. indirect utilitarianism).
That's my account. Hilzoy has promised the next post in her series will tackle the question of "which set of rules is just?", so she might offer her own version there. Though it sounds like it may instead focus on assessing particular proposed rules, rather than this "meta" issue of what determines the normative status of institutional rights. I guess we'll have to wait and see...