This time I'm quoting from AC Grayling's Towards the Light: The Story of the Struggles for Liberty and Rights (Boomsbury, 2007), pp. 260-61:
Critics have focused on the arbitrariness of the claim that nature or a deity has somehow magically endowed people with rights to life, liberty, property and happiness, when in fact the idea of these things is a human invention, and their existence as rights (in those dispensations where they indeed are rights) is the result of decisions to regard them as such. I call this the "arrogatory theory of rights"; experience and rational reflection show what is required to give individuals the best chance of making flourishing lives for themselves, and these framework requirements we institute as rights in order to make the chance of that flourishing available.
So we do not need to be wedded to the idea of "natural" rights at all. The proceeding just described - of people coming to see what they should lay claim to as the basis of their social and institutional arrangements - is acceptable and justifiable on its merits; one can make out an excellent case for saying that over time a consensus has been reached about the kinds of basic laws and principles required to so arrange things for individuals and groups that they can exist in ways we ("we" in those societies) recognise as desirable. It is for this that a robust and generalised concept of liberty is needed. As has often enough been pointed out (for example, by the philosopher H.L.A. Hart), if any idea of rights is to have content, the basic one must be liberty, for without it none of the others can apply.