Natural Law Theory is a Kind of Divine Command Theory

Natural Law Theory holds that beings such as us have ends from which our obligations may be inferred. I'll give you an example.

Suppose a natural law theorist holds, as he is apt to do, that homosexuality is wrong because heterosexuality is our natural purpose. This is to say that our natures are such that homosexuality is unnatural behavior for us and therefore immoral.

The obvious and common retort is to charge natural law theory with the naturalistic fallacy. This is to charge that it invalidly infers from the fact that something is unnatural in some biological sense to the conclusion that it is wrong. The reply from the natural law theorist is that the inference is indeed valid because here by "biological" we include the notion of God's creation.

However, this reply is just to say that we may infer our moral obligations from what is natural because God has chosen which things to make the natural things.

Another example will help. A natural law theorist is liable to argue that a human embryo is a rational being and therefore, in spite of the fact that it has not brain, has a mind and a right to be treated as any human being with a mind ought to be treated. His reasoning is that the embryo is rational in that its nature has been selected to be such that it will someday reason. His reasoning is not that the embryo has a genetic tendency to go on to become rational. The difference between these two kinds of reasoning is that the natural law theorist's reasoning assumes that God's choosing a thing's natural tendencies give that thing its end. In other words, it is only because of God's choice that the being in question has such and such an end. That is the Divine Command Theory.