| The idea of natural law is so deeply ingrained in our methods of ethical and moral inquiry that, like mythology, it consistently turns up in every culture I've ever studied. Unfortunately, natural law is subject to certain logical fallacies.
The Argument
Let's take a look at the natural law argument in its pure form. Any specific argument I take is bound to be contravertial, because natural law arguments have a nasty habit of eliciting opposite arguments that also take the "natural law" form. Nonetheless, let me take one that is seemingly commonsensical.
1: All people have certain rights, respect for which is morally binding. This is a natural law, as true as the laws of thermodynamics.
2: We are capable of violating people's rights.
3: Sometimes it feels good to do things which violate people's rights.
4: Violating people's rights is always bad.
Therefore, if we maintain all of these beliefs, to be consistent we must forgo certain pleasures so as not to violate other people's rights.
The Problems
Now, this way of arguing porbably looks familiar. And it's a nice little argument. But there are several problems here.
1: It's not "binding." What kind of "law" is this? What does "morally binding" mean? The "laws" of thermodynamics are "binding" in the sense that it is impossible to violate them; -- there may be ways to sidestep them, but not to actually break them. However, it must be admitted that it is possible to violate people's rights. If it weren't possible, there wouldn't be a problem. In fact, the whole problem of violated rights exists because "respecting rights" is not a universal law.
2: Appeal to Epistemological Privilege. What makes you so sure that this is "The Good?" Even if the "natural laws" were of a different sort than the "physical laws," how do you discover them? More to the point, what's so special about you that you know these "universal forms" and yet, nobody else does? This is Proof by Assertion, also known as Begging the Question. Everyone who comes up with a "natural law"-type argument always posits faculties ("reason," "intuition," etc.) whereby we can discover these faculties, and usually the problem with these faculties is that they are underused. After all, if everybody knew them, there wouldn't be a problem. In fact, the whole problem of violated rights exists because some people don't believe or don't care that it's the "way of things."
3: False Foundation. This is often referred to as the "Is-Ought Problem," and its identification is usually attributed to David Hume, though I think it actually originally comes from Immanuel Kant (who also committed it, for that matter). The idea is that you can't determine what ought to be with reference to what is. If you've already presumed that the real world is defficient in some way, how can you cite naturalistic evidence to point towards perfection?
If it offended the ultimate fabric of existence when we act in a certain way, it would not be possible to act in that way. This is by analogy to the laws of physics. If we can act that way, then it is not prohibited by the ultimate fabric of existence. Therefore, some other basis of criticism must be found whereby the action is not considered as "bad in itself," and goals are not considered as "good in themselves."
I'm a philosophy student, and I intend to go into the field of ethics. Therefore, I see this problem all the time. I've never, ever seen a philosopher who didn't commit the natural law error at least sometimes. Lao tzu, Chuang tzu, Friedrich Nietzsche, the Marquis de Sade, and Abraham Maslow only commit it in a way that is tangential to their real philosophy; their main message is too solid to be challenged on these grounds. I'm not familiar enough with Hume to comment about him.
Kantianism, utilitarianism, Objectivism, Christianity, Judaism, Wicca, Buddhism, ad infinitum, all fall prey to this way of thinking. I consider it to be semi-mythological, and therefore both an impure manifestation of mythology and an impure manifestation of philosophical thought. We all want to feel like we are part of a natural harmony; however, we cannot use logic to justify this.
On the other hand, its nontruth cannot have any bearing on our actions, either. | 
