1. There must be an objective moral law; otherwise: (a) There would not be such great agreement on its meaning. (b) No real moral disagreements would ever have occurred, each person being right from his own moral perspective. (c) No moral judgment would ever have been wrong, each being subjectively right. (d) No ethical question could ever be discussed, there being no objective meaning to any ethical terms. (e) Contradictory views would both be right, since opposites could be equally correct.
2. This moral law is beyond individual persons and beyond humanity as a whole: (a) It is beyond individual persons, since they often sense a conflict with it. (b) It is beyond humanity as a whole, for they collectively fall short of it and even measure the progress of the whole race by it.
3. This moral law must come from a moral Legislator because: (a) A law has no meaning unless it comes from a mind; only minds emit meaning. (b) Disloyalty makes no sense unless it is to a person, yet people die in loyalty to what is morally right. (c) Truth is meaningless unless it is a meeting of mind with mind, yet people die for the truth. (d) Hence, discovery of and duty to the moral law make sense only if there is a Mind or Person behind it.
4. Therefore, there must be a moral, personal Mind behind this moral law.
Stentor does a very good job at dismantling a few of the weak spots in the argument, but one he doesn’t touch is the shift from moral agreements to moral disagreements. People agree on morality a lot, but they disagree just as much. Different moralities consider one another immoral as a matter of routine. And a good rule of thumb is that any argument that uses for support both a thing and its opposite, as with 1a) and 1b), is worthless.
Also shockingly absent from the argument Carter reproduces is any discussion of where moral perceptions might actually come from. If you accept the basic idea of evolutionary psychology, then morality is just an evolved response to paleolithic conditions. If you want to put a positivist spin on EP, then it’s updated as needed by modern societies; I can’t help it but notice that racism went out of vogue around the same time it became a liability in a globalizing world.
While Stentor already addresses the “Morality is beyond humans” part, there’s another complicating factor he doesn’t talk about: hypotheticals. What’s moral is based on the fact that we’re social animals that reproduce sexually. It wouldn’t apply to sentient plants, or to sentient animals that reproduce like the mites that are devoured from within by their daughters. It wouldn’t even apply to an isolated human society with only 50 members, who must plan their reproduction in a way that minimizes incest in order to avoid dying off due to recessive genes’ being expressed.
And finally, laws don’t have to come from legislators. No celestial legislature voted on the law of gravity. No intelligent designer invented the principle of natural selection. And no economist-king mandated by fiat that trade offer comparative advantage to both partners.
So a non-shoddy version of the argument would be, “There’s some conventional morality, based on the points people do tend to agree on. This morality is beyond individual persons, but is too tightly coupled to humanity’s current condition to be really universal. Like social science, it is sort of an emergent property of socieities, rather than an explicit writ. And it certainly didn’t come from any deity.”