I like to begin with what we know best.
We are taught morality as children. I know, because I did this with my kids. Some of the rules we teach they eagerly accept. My children did not need to be taught that things ought to be fair. They believed this deeply and unquestioningly from the time they could talk. They disagreed about whether it meant they had to share toys with each other, but they accepted (indeed insisted) that favouritism was wrong. Admittedly, they were much less insistent about it when the favouritism was in their favour.
Other rules require more work. Don’t hit other people, don’t lie, play nice. Teaching children, the boundary between ‘morality’ and ‘politeness’ and ‘social expectations’ is blurred to invisibility. They must be taught how to behave so as to fit into society: they must learn norms.
To get a child to obey a norm they don’t already accept, you have to appeal to something else they already value. They accept that they should brush their teeth because they don’t want to get cavities, which they are told will hurt and result in a dentist visit. They accept that they shouldn’t hit other people, maybe because they accept that they wouldn’t like to be hit themselves if the situation was reversed, but very definitely because they do not want to be punished for hitting, which they will be. Reasons and evidence are sometimes useful; punishment and rewards work much more reliably.
For all that kids have to learn so much before they are civilized, their moral intuitions develop unpredictably. My daughters have quite different personalities, and that carries through to their moral intuitions. The eldest is much gentler and more forgiving, the younger leans into retributive justice. Where this difference came from I don’t know.
These differences impact the children’s motives. Some actions one would approve, the other would reject. I see this all the time in their squabbles.
When we look at the world we live in, we observe morality everywhere in human life. It is both internal/psychological, and external/social. Psychologically we observe that people are sometimes motivated to act based on moral considerations: these moral considerations are a species of value, preference, desire. Because my children value animal lives, they do not wish animals to suffer, and they act so as to help animals when they can. They are conflicted over spiders in the house, however — these days they embrace a savage insistence on arachnid extermination except when I am available to safely put the spider outside.
We all murder mosquitoes. Without hesitation or remorse.
My kids would not hesitate to externalize many of their moral preferences. They believe that discrimination is wrong and condemn racism and intolerance. Nobody should be racist, nobody should be a bully, nobody should murder or thieve. People who do these things should be stopped and punished. So they suppose that everyone else should obey the moral principles they have accepted, and that punishment is warranted for those who fail to obey.
So far so good, so easy. Most people in most places are like this: they accept and are motivated by various moral rules, some of which they externalize and demand be enforced by society. While there is vast disagreement about what rules they should be, every society enforces moral rules; sometimes legally, sometimes in other ways.
The moral subjectivist says: this is all there is to morality. It is people having various preferences and values, and instituting social rules to promote them.
The nice thing about relativism is it identifies morality with things that clearly and observably exist. It does not propose or presuppose anything else. In one sense, it is obviously correct: morality does refer to the social rules that are used by people and societies to guide their conduct. The question is whether there might be another sense, that refers to something more.
Moral realists suppose that, in addition to the observable facts about what people believe and the moral rules that society imposes, some of these rules are true and others false in an objective sense.1 A society could promote a set of rules, but in this bigger sense some or all of these rules might be objectively wrong. There is an intuitive appeal to thinking moral rules can be objectively true or false, since we each2 consider the morals we approve of to be correct and morals that conflict with them to be wrong, and it is very natural to express this in the language of truth and falsity. It is natural for people to say things like ‘it is false that intentional killing is always wrong. What about war, or self-defense?’
I am a philosopher, though, so I’m not easily persuaded by such trifles as ‘how people talk’ or ‘what they think’. There are different ways of expressing the anti-realist response to this kind of talk; what follows is just mine, although I don’t think the differences are very important in the end. The core idea is just that there aren’t any standards, values, or rules outside the ones humans have come up with. Consequently, arbitrating between conflicting moral systems, while possible insofar insofar there is some overlap, has no outside standards to appeal to.

Consider: when we have two incompatible moral values or rules, how do we decide which is right? Many realists, with a self-confidence I find astonishing, insist that we simply intuit the correct answer. The problem with that is people have conflicting intuitions. And not just people. Aliens, or tigers, or people from very different cultures than our 21st century modern western one, intuit different things. Pressed on this, realists retreat to claims that some people are morally defective and don’t have the correct intuitions. How can we know which people are the defective ones? Is it majority rules? What if most of the society supports slavery, or infanticide, or human sacrifice? There have been plenty such societies. Our society is arguably engaged monstrous and indefensible acts of animal cruelty. Does the fact that most people don’t intuitively believe that insects are worth moral consideration mean that Bentham’s Bulldog has defective moral intuition?
I mean, yes, clearly, but you’ll never be able to persuade him that. And how could you? You’re opposing your intuitions to his.
This is not to say that nobody can be persuaded of anything, morally. Lots of moral beliefs are tied up with factual
beliefs. If you are arguing with a Marxist, you can talk with them about the history of communism, and that might (and in many actual cases did) cause them to abandon their Marxism. The fundamental moral commitment of most Marxists is not actually to Marxism, it’s to making everybody happier and increasing equality. So if Marxist states don’t actually do that, Marxism loses its appeal to such people.
Not only that, but people often hold moral commitments that can conflict with one another. I believe in promoting public welfare but also in free speech. You might be able to persuade me to censor something, despite my belief in free speech, if you can convince me it’s going to cause huge social harms. Or maybe not, if I think the importance of the freedom to say those kind of thing outweighs the social harm, or the censorship would cause even worse social harm.
Moral realists try to finesse the impossibility of getting a complete consensus on what is intuitively right ot wrong by looking at extreme examples in the hope of getting agreement on at least something. Smashing innocent babies in the head with hammers is a popular case. But of course many people have done that and worse, convinced that they were doing the right thing. How do we know they were objectively wrong? What is the difference between our intuitions and theirs? History is not short of examples of self-righteous atrocity. The realists simply refuse to grapple with someone who will not agree with their own historically contingent and situated personal intuitions. They presume that they can find some common ground with their audience; we know that they need not.
Some might suggest that revealed religion is a route to moral certainty. For those who accept a revelation, it might suffice to ground one set of rules as objectively true. But for those of us on the outside it raises the question of how we know which revelation to go with. One also wonders why one is obliged to accept the morality of one’s creator, just based on His say so. Shouldn’t the logic of God’s word be self evident? Ultimately divine revelation has the exact same problem as intuition, given the plethora of revelations on offer. History shows that human societies love to tie their morality to religion, and commonly fail to thereby persuade their neighbours of the truth of either.
Philosophers have also tried to derive morality from pure logic without relying on intuitions. Here's a fun recent attempt, and a response that could have been much shorter, since the first point is decisive. You can’t derive morality from pure logic, because logic doesn’t have any moral content, so any substantive moral axiom is contestable. Nor does analysis of the bare concept of ‘action’ get anywhere at all. Sorry, Kantians. I kinda wish there was more to say about this, but Kant’s moral arguments suck.3
So we have two accounts: one that is clearly true as far as it goes, and another that cannot establish the basic facts it relies on to function. We observe moral systems in the real world, a diverse plethora of human created systems that irreconcilably differ on key values and rules. That is morality, as the moral relativist observes and understands it. There is also the morality of the philosophers and prophets, the hypothetical objective and transcendent system of rules that bind all agents. Setting aside whether there is evidence such a thing exists, we don’t have any way of establishing what those rules are. I suggest that therefore moral relativism is the default position we should accept, in the absence of a clear explanation of how we could possibly establish the truths of the alleged objectively correct morality. Intuitions won’t do it and it can’t be pure logic. Any other contenders?
By contrast, the subjectivist says that these claims are true relative to an existing moral framework. So “execution is wrong” is true according to my moral framework, but false according to Gerorge W. Bush’s. There is no objective standard to evaluate it by.
Except for a few philosophers.
Kant is famous because he told people what they wanted to hear using arguments they couldn’t understand. Or maybe I’m just an idiot for not seeing why egoism is self-contradictory.
Music to my ears. Thank you for setting the record straight. It’s so satisfying when someone puts into words exactly what you think so concisely, persuasively, and with such razor-sharp clarity. I wish I had a million dollars lying around to give you. Listening to people argue for moral realism has been just driving me crazy. Indeed, I started to think that I was crazy because I couldn’t put into words my very strong intuitions as to why moral realism is just an absolute nonstarter as a theory. You’ve given me those words and I’m so, so grateful. 🥲 Thank you, and may you succeed in all your future endeavors!
Always good to see more people pushing antirealism. However, I think you give realists too much credit. You say:
"There is an intuitive appeal to thinking moral rules can be objectively true or false, since we each2 consider the morals we approve of to be correct and morals that conflict with them to be wrong, and it is vary natural to express this in the language of truth and falsity. It is natural for people to say things like ‘it is false that intentional killing is always wrong. What about war, or self-defense?’"
An intuitive appeal to who? I don’t find this intuitive or appealing, and I don’t think most people do, either. I don’t think my moral standards are “correct” in an objective/stance-independent way, and I don’t think typical English treats moral claims in a presumptively realist way. I don’t know how other languages treat moral discourse, if they even have it. That’s an empirical question.
I checked endnote 2. It says "except for a few philosophers." But ... how do you know it's just a few philosophers? Whether most people find realism intuitively appealing is an empirical question. At present, there is very little evidence to suggest most people do, and the more rigorous empirical studies have found that a majority of respondents favor antirealist responses to questions about metaethics. I think there are methodological concerns with these studies, but even so, what we do not have is any good evidence that most people find moral realism intuitively appealing.