There are a few moral ideals that are common to all social groups, such as not stealing or committing senseless murder. These have lead to many people, both religious (theist and deist alike) and nonreligious, supposing the existence of moral absolutes.

These generalized moral ideals are picked up on keenly, but little attention is paid to the fact that other than these few things, all other moral ideas are blurred, subjective, and mutually exclusive. For example, some cultures seen cannibalism as a moral duty, and other see it as the worst imaginable crime.

The common morals (not stealing, not fruitlessly murdering) can be traced logically back to evolution via natural selection, and that is the reason that they are the only ones truly common to all social groups. They are the morals that help people live together as a group, which in turn helps them to survive as a species. It boils down to basic common sense: if you want to have a successful group, you can’t have people stealing from one another and you can’t have people killing at random.

All other morals enter the realm of subjectivity. Every culture agrees that a baseless random murder is wrong, but they disagree severely over definition of “baseless,” when it is okay to take a life and when it is not. Just a few hundred years ago it was socially acceptable for a brother to murder a man who had sex with his sister out of wedlock. Many countries today still practice honor killings where it is morally justifiable for a husband to kill is adulterating wife, or a father to kill his disobedient daughter. Among the Asmat in New Guinea, before they were influenced by Western society, it was not only considered correct, but a moral and religious obligation to kill and cannibalize your enemy.

Our culture today says that murdering the man who had sex with your sister is wrong, but killing another man during a war is alright. What a culture defines as “baseless” can change from era to era and culture to culture, but the idea that baseless murder is wrong remains. However, baseless killing, although universally agreed upon by human cultures, is not a moral absolute because of the way it fluctuates–that ever-changing definition of when it is okay to kill and when it is wrong. There are no moral absolutes, just generalizations.