Below are a few paragraphs I recently wrote for a book chapter, explaining why the concept of natural law is both rhetorically attractive and pernicious. They will likely be cut from the chapter because of space limitations. So, I thought I would post them here (for possible use later):
John Locke first argued that individual liberty and property are rights under "natural law." Each person owns herself, and because each person requires sustenance to survive, so her acquisitions must become her inviolable "property" (so long as she doesn't claim too much and leaves "as good and enough" for others to claim). Some liberal thinkers, especially self-described libertarians, continue to embrace Lockean “natural law” on behalf of individuals and their property. But they are not the only theorists that claim the mantle of "natural law" for their preferred political values and policies.
Many progressive and conservative scholars also subscribe to theories of “natural law,” which are, more often than not, at odds with libertarian “natural law.” Progressives agree with libertarians that liberty and property natural “human rights,” but deny they are the only human rights. So too, they claim, are rights to social security, equal pay, employment, holidays, child care, and others set out in the 1948 United Nations Declaration of Human Rights. Many of these so-called “positive” rights require state action that would redistribute resources via taxation and social programs that inevitably would violate individual liberty and property rights. Consequently, the progressive “natural law” of human rights violates libertarian “natural law" (though not other liberal theories that either do not rely on "natural law" or have a more relaxed view (as Locke, himself, did) as to what constitutes consent to interference with liberty or property rights.
Theocratic “natural law” theories are no less inconsistent with libertarian “natural law.” The basic purpose of religious doctrine is to propagate moral constraints on individual liberty, under threat of divine punishments that often are meted out by humans. In ancient times, some religions required human sacrifice, among other rituals. Even in modern times, the three major monotheist religions impose various requirements that conflict with individual liberty, including prayers, tithing, dietary restrictions, and prohibitions against certain consensual sexual relations. Such requirements would not trouble libertarians so long as individuals remain free to choose and change religions. If an individual chooses a religion that constrains her liberty, then she consents to the obligations imposed. For most of human history, however, freedom of religion did not exist. In fact, it was religious persecution, more than anything else, that gave rise to liberal philosophy in the eighteenth century.
Religious convictions (or confessions) can be imposed by social norms as well as by law, which makes them no less coercive. Whether sanctioned by the state or imposed by social norms, coercive mandates violate the liberty and property of individuals, especially of non-congregants or non-believers who are persecuted. The arch libertarian Murray Rothbard (1999 ) illustrated the counter-libertarian nature of religious rules by reference to early Puritan Church doctrines in Massachusetts. The Puritan churches had rules against kissing a wife in public on the Sabbath, mandatory church attendance enforced by either local police or other congregants, prohibitions on the playing of shuffleboard and other games in public houses, and punishment of idleness and drunkenness (but not drinking). Rothbard deplored those coercive rules, as well as the Puritans’ treatment of women as “instruments of Satan." He did not bother to mention witch-burning as a problem.
It would be surprising, indeed, if theologians did not invoke “natural law” to support their preferred rules for ordering society. Unsurprisingly, different religious denominations do not agree on a single, consistent set of "natural" laws. They may overlap on certain specific rules, such as the golden rule and the golden mean, but the sets of "natural" rules they provide differ significantly. And none of them is consistent with either the progressives’ “natural law” of human rights or libertarians’ “natural law” of individual liberty and property. So, which is the real “natural law?”
Imagine a good-faith debate between natural law theorists promoting different rules. One claims the "natural" rule is x. The next replies that it is y. A third responds that it is z. Each of them supports her claim by pointing to some (for them) authoritative source, e.g., a passage from the Old Testament, the New Testament, the Quran, the Hindu Ordinances of Manu, John Locke's Second Treatise of Civil Government, the US Declaration of Independence, a Kantian categorical imperative, and the list goes on. How might the dispute be settled? It would not depend on factual evidence, of course, because facts about rules have no bearing on whether a particular rule is "natural."
We also have to consider the possibility that no one’s claims of “natural law” is correct. The founder of utilitarianism, Jeremy Bentham, famously dismissed the "natural law" claims of the French Declaration of the Rights of Man (authored in large part by Thomas Jefferson) as as not just nonsense but “nonsense upon stilts.” In the nineteenth century, Sir Henry Maine (1901  at 74) observed that “the importance” of natural law theory “to mankind has been very much greater than its philosophical deficiencies would lead us to expect."
So, does natural law exist and, if so, can we possibly know which version of it is correct? The answer to the first question appears to be that we cannot know because there is no fact of the matter. "Natural law" is an assertion of belief not of fact. It is an inarguable super-normative claim about what rules should be in effect. As such, the compound term "natural law” has great appeal as a strategic rhetorical device. Whether it is anything more than that is unknowable and somewhat beside the point. Claims of "natural law" really are not even "arguments" about which rules should be followed. They are argument-enders. Good reasons may support an ordinary law, but that law is, by definition, inferior to a "natural law," period. For this reason, arguments from "natural law," including libertarian claims of "natural law," might be said to be fundamentally illiberal.