Natural Law Theory
Perhaps the final nail in the coffin of Divine Command Theory is the authority St. Thomas Aquinas brings to bear in abandoning it. Aquinas, of course, 'got' the Euthyphro Argument and understood that moral theology requires much more subtley and sophistication than Divine Command Theory's "do it because I said so!" approach.
Natural Law Theory is a fascinating alternative to Divine Command Theory. It assumes the Teleological World View, which is the view that the world and each thing in the world has a purpose or function. A teleological explanation necessarily mentions the purpose or function of whatever is being explained. I find it is easiest to understand the concept of a teleological explanation if we use human artifacts as examples.
What is a watch? It is a device for keeping track of time. If you want an explanation of what a watch is, you refer to its purpose or function. Of course human artifacts have functions or purposes; these are the functions or purposes we humans give them.
According to the Teleological World View, every object in the world has a function or purpose. Thus a tree, a rock, a river, and each granule of sand on the river's bank has a purpose or function.
To modern ears this is a peculiar World View. It just seems odd to say that the rock in the field has a purpose. Certainly we might put it to some purpose or other. But that is not to say that the rock itself has a purpose. The rock itself is just the result of perfectly impersonal geological forces which have nothing to do with intentions or purposes. Similarly, Darwin has taught us that flora and fauna, ourselves included, are the result of environmental factors which have nothing whatsoever to do with purposes.
On the other hand, the teleological world view can be appealing, particularly when we are confronted with tragedy or discomfort which we have a difficult time rationalizing or understanding. For example, one frequently hears phrases like
There's always a reason...
It happened for a reason...
Who knows why it happened, but someday we'll understand...
There's a purpose to this, we just may not know what as yet...
To be sure, these and proclamations like them are most often heard in our hours of desperation. Somehow it is supposed to comfort us that there is a purpose. Taking the Teleological World View seriously thus presents the framework for an interesting alternative to Divine Command Theory.
Natural Law Theory holds that actions are morally right just in case they are consistent with all relevant proper purposes. If an action somehow violates a relevant proper purpose, the action is morally wrong. The best examples of this come from the Catholic Church's interpretation of Natural Law Theory. (Perhaps there are other interpretations?)
According to the Catholic Church, actions like masturbation, the use of contraception, homosexual sex, and abortion are morally illicit--morally impermissible--because they violate or contravene the relevant proper purpose of sex which is, by the Catholic Church's lights, procreation.
A good question is why something like sex must have a proper (singular) purpose. It seems plausible to hold that sex has many purposes, only some of which are directly relevant to procreation.
The upshot is that Natural Law Theory passes the Standard of Clarity only if we have an interpretation available like the Catholic Church's to tell us just what to count as relevant proper purposes. However, any interpretation which professes to establish all relevant proper purposes must be arbitrary since it is impossible to adequately define "proper purpose" in such a way that we can distinguish proper purposes from purposes generally. That is, we cannot provide necessary and sufficient conditions on being a proper purpose, so we conclude that Natural Law Theory fails the Standard of Clarity.
Worse, Natural Law Theory also fails the Standard of Coherence because it is externally incoherent: the Teleological World View is false. Or, as I sometimes like to put it, the Teleological World View is a Peculiar World View, since it has us ascribing non-existent properties to objects in the world: most notably, the property of having a purpose or function does not exist for natural objects.
Moving on, Natural Law Theory arguably fails the Standard of Reflective Equilibrium. Making use of the Catholic Church's interpretation in particular, it can be argued that the implications of Natural Law Theory with respect to homosexuality, contraception, and masturbation are pre-theoretically, reflectively, and with many good reasons, intuitively false.
The weight of argument drives us to reject Natural Law Theory as a possibly true theory.