Objectivity
Thomas Jefferson wrote, “A free people claim their rights as derived from the laws of nature, and not as the gift of their chief magistrate.” Note, Jefferson did not say a free people claim their rights as derived from a Constitution, for the rights enumerated in the Constitution must be a reflection of the rights found in natural law. When you ask people about equality, they often reply, “we’re equal before the law,” and they’re right, we are equal before the law according to the Constitution. However, the real question is, are we equal in reality, as a law of nature, which the Constitution then reflects and secures as law?
Subjectivity
A negative answer to the latter question lands morality in subjectivity. Subjectivity is the basis for might makes right, which runs counter to a rationally based ethic; an ethic which all persons are rationally obligated to uphold, and which forms the basis for freedom and true happiness. The logic of subjectivity goes like this, If value and morality are purely subjective, that is, exist only in your head and not as a reflection of reality, then when you say that such and such is wrong you are really saying you feel or imagine such and such is wrong *though it's really not*. The 'really not' logically accompanies every expression of a subjective moral view. To be clear, when I say 'really' I mean 'in truth', and I accept the classic definition of truth: 'the conformity of the mind to reality.' Therefore, to take the subjectivist line looks like this, in real terms: "I feel the holocaust was wrong, but it really wasn't." Or, "I think dragging homosexuals behind my car is wrong, but it's really not." This is monstrous thinking, and it’s patently false.
Natural law
Invariably, discussions about the natural law produce some form of this common response, “but desire x IS natural because people are born with the inclination; plus, such desires exist in the animal kingdom”. However, if natural law is to mean anything, then clearly we cannot say that just because a person is born with a certain tendency that therefore it is natural; likewise, we cannot point to animals and say that what is natural for them is natural for us – clearly, we cannot do this.
No, the meaning of natural law -- what Thomas Jefferson and Thomas Aquinas, and other great proponents of natural law knew it to mean -- starts from the premise that man is a rational animal, that it is part of our nature to rationally govern our mere animal desires according to an outline, an over-all goal. The rational part, if you’ll notice, allows us a certain insight into the skin, so to speak, of other rational animals – we can speak for other people, for our rationality is in some way common. For instance, according to the self-evident principles of rational thought, we can say that any given person is in error if they state that a finite part is greater than the whole of which it is a part; likewise, if another person affirms that two and two equal six we can speak for them and say that they are wrong.
However, many people want to treat morality as if it somehow escapes our ability to speak from within each other’s skin; but this is simply nonsense. There are certain things we can say with certainty about other human beings concerning moral choices. We can say, for instance, that acting on the anorexic aversion (not acting to eat normally) is bad for human beings, but we can also say it is wrong. It is wrong for you to starve yourself, because it is wrong for me to do so; since we share the same essential nature I cannot say that something, which adversely affects what is essential to my being, is ok for you, since it effects what is conceptually indistinguishable from my own nature (your essence).