To begin with, recall that conventionalism is a relativist theory of ethics that maintains that what is good is determined relative to a society, convention, or culture. On this account, no society or culture is better than another.
A popular way to support conventionalism is to appeal to the cultural differences argument. The argument goes like this.
This seems like a powerful argument. But let’s look a bit more closely. How might you start to evaluate this argument?
Let’s say you start by asking if it’s valid. Recall that an argument is valid if, assuming the premises are true, the conclusion is also necessarily true. The cultural differences argument is valid. This is because, if we assume (1) cultures wouldn’t disagree about morality if there were universal ethical truths, and (2) that cultures do in fact disagree, then it follows logically that there are no universal ethical truths.
Now let’s ask if its sound. Recall that a sound argument is one that is both valid and all the premises are true. The second premise (different cultures have different moral values) seems true. But the first premise is more questionable. In the next section we will see why.
The first premise of the cultural differences argument basically says moral disagreement shows there isn’t any objective morals that we could all agree to. But this doesn’t seem true. That’s because we can disagree and there still be a truth of the matter.
Imagine that we disagree about how the universe started. Let’s say I think the universe has always been there and so doesn’t have a beginning; whereas you think the universe began with a big bang.
Just because we disagree, it doesn’t mean there isn’t a correct answer to the question whether the universe had a beginning or if it’s always been there. We just don’t have conclusive evidence yet.
The fact that we disagree doesn’t necessarily mean neither of us has gotten the objective truth. It can mean that just one of us has failed to get the objective truth. In this case, we don’t have to accept that disagreement is the end of the story. We can still hold open the idea that one of us is right and one of us is wrong.
As you can see, the conventionalist is wrong to think disagreement means there is no objective truth. Even when it comes to very difficult issues that can’t be answered, disagreement still doesn’t mean there isn’t objective truth.
Again, this shows that conventionalism is wrong to think disagreement establishes there are no objective truths. And this means that it’s also wrong to say that anyone’s view is as justified as anyone else's.
One of the consequences of what we’ve said, is that a culture isn’t necessarily justified in its beliefs. A conventionalist might give a very accurate picture of the morals of a specific culture or society, but it doesn’t mean that these morals ought to be followed.
Today, most people would say that this was wrong. In a similar way, it used to be thought that poor people shouldn’t be allowed to vote, but now this is widely rejected. Just by pointing out differences in ethical views, then, you can’t justify those views.
A final possible problem for the conventionalist is that it appears there are in fact some agreements across cultures. For instance, most societies say that killing is wrong. But a conventionalist might say examples like this don’t count. They could argue that the prohibition against killing isn’t an ethical truth, but just a way for humans to survive.
If the only reason humans don’t kill each other is to survive, it also doesn’t seem like an ethical decision. Rather, it seems like it’s just something that we do as the kind of animals we are.