The first candidate for the source of morality is culture. It’s very common for people to believe that social conventions create morality, and that their culture determines what is permissible and impermissible by a sort of unspoken consensus. People who have this view think that whatever most people in their society think is permissible is—for that very reason—permissible, and likewise whatever most people in that culture think is impermissible is thereby impermissible.
This view is called Cultural Relativism, the position that morality is relative to one’s culture. Cultural relativism is a moral theory. It’s a theory, in the scientific sense that it is a system for explaining how morality works, the same way that quantum theory is a system for explaining the behavior of quantum (subatomic) particles. While someone who holds the view that culture is the source of morality would definitely be a cultural relativist, it’s not the case that each source of morality has a corresponding moral theory. Some sources of morality can lead to more than one moral theory, and some moral theories can be traced back to more than one source of morality. Don’t worry though, we’ll go one at a time to keep it simple.
Now, a confession: there is a reason why we’re discussing cultural relativism first. It’s because cultural relativism is nonsense, and though it might be the most commonly held view by people who have never studied ethics, it is the least plausible explanation of how morality works, and possibly not a real moral theory at all. We’re discussing it first so we can eliminate it and move on to more sensible theories.
“But wait!”, you exclaim, “Cultural relativism seems like a perfectly reasonable view, and if so many people believe in it, they can’t all be wrong!” I understand your concern. You were raised in a society that defends diversity and respects people’s cultural differences. Everyone knows that we should respect different cultures and their moral beliefs. You might think, “if they believe something is permissible, then who are we to tell them they are wrong?”
Firstly, yes, they could all be wrong (remember Copernicus), and secondly, because a lot of people—possibly a whole culture of people—could all be wrong, we do not have to respect every cultural difference. In fact, the reason that most people believe they are cultural relativists is actually the exact reason why they are not. Most relativists think that they should respect other cultures and not judge people of other cultures by the standards of their own culture. But this shows that there is one thing that relativists think is not relative: respecting other cultures. If it is always morally obligatory to respect other cultures, then the idea of respecting other cultures must not come from culture. It must come from one of the other sources of morality.
So why would smart, well-meaning people defend cultural relativism? Often, the people who think that they are cultural relativists are anthropologists or sociologists, people who study how cultures and the people in them behave. These are intelligent people who have simply made one key mistake. They assume that what people actually do is the same thing as what they should do. But when you think about it, that assumption makes no sense at all. People do things that they shouldn’t do all the time. To think that people should do whatever they actually do, and furthermore that people should do things because they actually do them anyway (the claim cultural relativism makes) is absurd. Another way someone might try to defend relativism might be to say that it’s not what people in a culture actually do, it’s what they believe. But of course, believing something is true doesn’t make it true, and believing something is morally permissible doesn’t make it permissible. So this defense of relativism is just as much a failure as the first.
Let’s use a real-life example that focuses on another aspect of cultural relativism. If cultures are morally justified in doing whatever they do because they believe it’s right, then we’d have to agree that this is true at all times. That is, whatever a culture thought was right at some time in the past, a cultural relativist would have to admit was morally permissible at that time. Even if that same culture came to change its views since then. The specific example of this I’m imagining is America’s history of slavery. At one time in American history, the majority of people in the culture thought it was morally permissible to own other humans as property, a proposition that we find morally repugnant now. A cultural relativist would have to admit that because Americans at that time thought slavery was morally permissible, it was. For a relativist, it’s not that slavery was always wrong and people just took a while to realize it. Cultural relativists have to admit that slavery was actually morally right in mid-nineteenth century America because it was commonly accepted.
Could a claim like “slavery is morally impermissible” be false at one time, and true at another time? Could a claim like “women should be treated equally to men” be true in America and false in Saudi Arabia? Can anything be both true and false at the same time? If it did, it would be nonsense, in a very literal way. We couldn’t make any sense of it.
There is one last point to make about cultural relativism before we move on. It’s not common to all people. If morality comes from the things that all people have in common, culture doesn’t really work. Obviously, not everyone has culture in common. There are lots of cultures that are radically different from each other. So as a system for deciding what is morally right and wrong, cultural relativism is not only unreliable, it is contradictory and ultimately nonsensical.