I just watched a debate between Sam Harris and William Lane Craig on whether the existence of objective morality requires a supernatural source.
It's truly amazing how often debates like this come down to a failure on both sides to define their terms and, further, to agree on those definitions. William Lane Craig and other Christian apologists often accuse the atheist of being unable to justify the existence of objective morals within their naturalistic world-view. The apologist will then discount any argument made for natural objective morality on the basis that it assumes goodness or badness or preferability of certain results like well-being or thriving of sentient creatures, when, without a supernatural law-giver the atheist has no foundation for any such statements. The atheist will progressively simplify his arguments, thinking that the apologist just doesn't get what he's saying, until he gets to the basic statement of something like "it is better for sentient beings not to be in absolute misery." But again the apologist says that he has no basis for this valuation. So the argument, having been reduced to absurdity, hits a dead end.
The problem from the apologist's side is that he is defining "objective morality" differently from the atheist. When he says "objective morals" he means something like "divine commandments". He argues that God is the embodiment of absolute moral perfection and therefore, any quality he possesses is objectively moral and any action that opposes the moral is objectively immoral.
Now obviously, since the atheist does not believe in God, he is not arguing for that definition of "objective morality". Nor would it be logical to argue that objective morality, under the apologist's definition, can exist in the absence of a supernatural God. But the atheist defines "objective morality" differently. The atheist's definition of "objective morality" is something like "universally applicable ethics". He argues for the application of "rule-utilitarian" ethics under which an action is moral if it is likely to cause the most well-being and/or least suffering among all affected sentient creatures and would do likewise if that action were taken as a guideline for what others ought to do in similar situations. So, for example, although it may cause more well being on an overall scale for me to go next door and kick my single neighbor out of his house by force and invite three homeless people to take it over, it would decidedly create a world of less well-being if that action were taken as a guideline for how one ought to act. Everyone would be kicking everybody out of their houses, personal property would become meaningless, etc.
Now this is simply a system of rational ethics which best correlates to the system of conscience that we all have within us, but it does give us an objective, rational method to arrive at a morality which is independent of culture, religion, personal wants and desires, etc. Therefore, it seems to be a pretty good exposition of what an objective morality could look like in a naturalistic world view.
The apologist's only possible argument to this is "well, what makes well-being good? You are arbitrarily setting up your system on the desirability of well-being and the undesirability of suffering."
Obviously, this is absurd, but that is where the defining of terms comes in. At this basic level, you do have to make the assumptions necessary in the definitions of your terms. Well-being is good because I am defining it as good, it has "well" in it. Suffering is bad because I am defining it as bad, what's more, everyone knows its bad, and if you don't accept that then the argument is doomed to fruitlessness anyway.
However, once you can accept that suffering is undesirable and well being is desirable, you have a perfectly legitimate non-supernatural basis for objective morality.
Furthermore, I defy any supernatural moralist to come up with an example of an action which is objectively evil that truly defies rule-utilitarian ethics; that is, an action that all moral people would agree is absolutely deplorable but causes widespread well-being and would do so if taken as a guideline for how others ought to act in similar circumstances. The fact that this can't be done argues that well-being and suffering are the true foundations for our moral systems.