One of the arguments moral relativists use to support their view that moral values are not objective is what I call the “change and diversity argument.” It is reasoned that since moral values have changed over time (we once thought slavery was moral, but now we don’t), and moral opinions even differ from culture to culture at the present time, morality cannot be objective.
This is not a good argument for several reasons. First and foremost, the presence of contrary opinions does not imply the absence of truth. Just because people disagree on what is moral does not mean moral values are not objective, nor does it mean that no one is capable of possessing knowledge of moral truths. Consider a mathematical problem posed to 10 students. If each student provided a different answer to the same problem, would it follow that no one was right or that there is no right answer? No. Relativists who offer the “change and diversity” argument against objectivism are confusing moral epistemology for moral ontology. While it may be that people can be mistaken about what is right and wrong, that no more implies that there is no moral truths than the fact that people get their sums wrong implies that there are no mathematical truths.
The second reason this is a bad argument is that it is not true that moral values are constantly changing or that different cultures have different sets of moral values. All cultures share the same moral values, even if they apply those values inconsistently, or disagree regarding the facts that determine when and how a moral value applies. For example, all cultures across time have agreed that it is wrong to take the life of an innocent human being, but they differ on the factual matter of who is human and/or who is innocent. We see this even in our own culture. Those who support abortion and those who oppose it do not have different moralities. We both agree that it is wrong to take the life of an innocent human being. Where we differ is on the facts. Those who support abortion think that the unborn are not human beings whereas those who oppose abortion do. There are no cultures in which love, kindness, and fairness are viewed as moral evils and cowardice, murder, and treachery are viewed as moral goods. The lack of moral consensus in the world is over facts, not moral principles.
While one would do well to explain these things to the moral relativist, Greg Koukl has come up with an easier and more tactical approach to accomplish the same result. Here is my adaptation of his approach.
To the moral relativist that offers the “change and diversity” argument, ask “Do you believe that science can provide us with objective information about the world?” Most would agree that while science can be wrong, it does provide us with at least some objective information about the world (e.g. the planets revolve around the sun). Continue on, “But haven’t scientists’ views about reality changed over time? Indeed, don’t scientists disagree among themselves on various issues even in our own day?” This is obvious. Continue, “Why is it, then, that you think scientific beliefs can be objective but moral beliefs cannot when there has been a history of change and diversity for both? You need to be consistent. Either diverse and changing opinions on a topic preclude the possibility of the existence and knowledge of objective truth or they don’t. You have to decide whether you are going to give up your belief that science can and does provide us with objective knowledge, or give up your argument against moral objectivism based on changing and diverse moral perspectives. Which is it?”
Hopefully this approach will help your friend to realize that the “change and diversity” argument is a bad argument for moral relativism.