I can't say enough good things about John Hasnas's paper Toward a Theory of Empirical Natural Rights. Hasnas argues, to my mind persuasively, that the set of negative rights to person and property familiar from the Lockean tradition emerged through centuries of human experience in Anglo-Saxon society. As Hasnas puts it, rights are solved problems -- in particular, solutions to the problem of minimizing violence in society. These rights emerged in something close to the state of nature (before a central government developed in England) and therefore are truly natural rights. A philosopher like Locke can come along afterward and tidy up the edges of the somewhat messy empirical rights as they emerged in social reality, but here again the philosophers are not creating a new concept ex nihilo but merely standardizing an existing social technology, as it were.