If moral conclusions cannot be deduced from facts about the world, the obvious next thought is that there are no moral facts for them to be about.
That is a serious position with serious defenders, and the previous lesson's argument leads straight to it. This lesson takes the anti-realist family at full strength, since a version stated weakly is easy to knock down and teaches nothing. It ends somewhere that surprises most people: whichever of these views is right, the reader still has to work out what to do, and by the same means.
The menu
Start by laying out the options, because the vocabulary is a mess and the same word is used for three positions.
Realism says there are moral facts, they are not constituted by anyone's attitudes, and moral claims are true or false by whether they match. It comes in a naturalist version, on which moral facts are ordinary facts about welfare or flourishing, and a non-naturalist version, on which they are a further kind of fact not reducible to any other.
Subjectivism says moral claims state facts, but facts about attitudes: "wrong" means "disapproved of by me" or "by my society". These are the relativisms of the previous lesson, and they are cognitivist, meaning that moral sentences say something capable of truth.
Error theory says moral claims genuinely try to state objective facts, and there are none, so all positive moral claims are false. John Mackie set this out in Ethics: Inventing Right and Wrong in 1977, and the position is the most radical on the list: torture is not wrong, and it is not right either, in the way that no witch is a real witch.
Expressivism says moral claims do not try to state facts at all. Saying that torture is wrong is more like booing torture than like reporting anything. A. J. Ayer's 1936 version was crude; Simon Blackburn and Allan Gibbard spent the 1980s and 1990s building versions that behave in argument almost exactly like realism, which is called quasi-realism.
Two axes cut across each other here. Does a moral sentence state something true or false, and is what makes it true independent of attitudes? Realism says yes and yes, subjectivism yes and no, error theory yes and there is nothing, expressivism no to the first question so the second does not arise.
The argument from disagreement, and why it is weak
The most popular argument against realism is that people disagree, endlessly, and that this is best explained by there being nothing to be right about.
Mackie's version, the argument from relativity, is more careful than the popular one. He does not claim that disagreement proves there are no facts. He claims that the pattern of variation is better explained by the hypothesis that moral codes reflect ways of life than by the hypothesis that they are imperfect perceptions of an objective order. People do not just disagree; their disagreements track their societies' economies and histories in exactly the way you would expect if the codes were adaptations rather than discoveries.
The standard reply has three parts, and it is strong. First, disagreement is compatible with realism everywhere else: physicists disagreed for centuries about the age of the earth without anyone concluding there was no fact of the matter. Second, much moral disagreement is downstream of factual disagreement. Societies that burned witches were not operating an alien value system; they accepted that maleficent supernatural harm was real and that those who inflicted it should be stopped, which is a value most modern readers share applied to a false belief. Third, agreement is broader than the argument suggests. Gratuitous cruelty, betrayal of the helpless and bad faith are condemned essentially everywhere, and the variation is in scope, in who counts, rather than in the underlying principles.
What that reply cannot do is explain away the residue. Some disagreements survive every correction of the facts, and Mackie's explanation of them is not obviously worse than the realist's.
Example. Two societies both hold that killing the innocent is gravely wrong. One practises infanticide of disabled newborns and the other does not. Is this a disagreement in values?
Not necessarily, and the distinction matters. If the practising society believes that a newborn is not yet a person, or believes that a disabled infant faces certain suffering and early death that exposure shortens, then the two societies share the principle and differ about its application, which is a factual disagreement about status or about consequences. Anthropological work on infanticide finds a great deal of this. What would be a genuine disagreement in values is a society that agreed on every fact, including the infant's status and prospects, and still held that its life carried no weight. Real cases are mostly the first kind, which is why the argument from disagreement is far less potent than it looks, and why establishing that a residue exists takes careful work rather than a list of shocking customs.
Now you. Someone argues that widespread moral progress, such as the abolition of slavery, proves realism, since progress requires something to progress towards. Assess it.
Answer
It assumes what it needs to prove. Calling a change progress is already a moral judgement, so the argument helps itself to the standard it claims to derive. An anti-realist describes the same history as a change in attitudes, driven by economics, literacy and the extension of sympathy, and can consistently prefer the later attitudes while denying that they are more accurate. What the history does provide is weaker and still worth something: the changes were driven by argument, by pressing people on the inconsistency between their principles and their practice, which is what you would expect if moral thought were answerable to something. That is evidence, not proof, and a quasi-realist will say their theory predicts it just as well.
Mackie's queerness argument
The stronger anti-realist argument is metaphysical rather than sociological.
Mackie asks what an objective moral fact would have to be like. It would have to be a feature of a situation that is both there in the world and, merely by being perceived, motivating: seeing that an act is wrong would have to move you not to do it, without any desire of yours being involved. Ordinary facts are not like that. Knowing where the door is does not make you want to leave. So a moral fact would be, in Mackie's word, queer: an entity of a kind utterly different from anything else in the universe, requiring a faculty of moral perception equally unlike any other.
There are two components, and they are worth separating. The metaphysical strand says such properties would not fit into a scientific picture of the world. The epistemological strand says we would have no credible account of how we came to know about them, and this is the sharper one. Evolution explains why we have the moral responses we have without any reference to their truth: tribes of cooperators outcompeted tribes of defectors, and that story is complete. Sharon Street pressed this in 2006 as the Darwinian dilemma. Either our evaluative attitudes are tracking mind-independent moral truths, in which case the realist owes an account of the coincidence between what selection installed and what is true, or they are not, in which case realism gives us no reason to trust any of them.
The realist replies are real but expensive. One is the companions-in-guilt move: mathematical and modal facts are just as queer, and nobody abandons arithmetic. Another is naturalism: identify moral facts with facts about welfare, which are perfectly ordinary, at the cost of the open question. A third is to deny the motivational requirement, holding that a person can judge an act wrong and simply not care, which is what the amoralist seems to be, and to let moral knowledge work like any other knowledge.
Expressivism and the Frege-Geach problem
Expressivism escapes queerness completely, because it needs no moral facts, and it runs into a technical problem that has shaped forty years of work.
If "lying is wrong" expresses disapproval rather than stating anything, what does it do inside a larger sentence where it is not being asserted? Consider the argument: if lying is wrong, then getting your brother to lie is wrong; lying is wrong; therefore getting your brother to lie is wrong. That is a plain modus ponens and it is obviously valid. But in the first premise nobody is disapproving of lying; the speaker is supposing it. For the argument to be valid, "lying is wrong" must mean the same in both premises, and expressivism seems committed to it meaning something different, since in one place it expresses an attitude and in the other it does not. Peter Geach pressed the point in the 1960s, building on an objection of John Searle's, and it is called the Frege-Geach problem.
Blackburn's answer is to build a logic of attitudes: to accept the conditional is to hold a higher-order attitude, disapproving of the combination of disapproving of lying while not disapproving of procuring it, so inconsistency becomes a kind of practical incoherence rather than a logical falsehood. Gibbard's is to model judgements as plans and treat inconsistency as the impossibility of a plan covering all contingencies. Both work, in the sense that they reproduce the inferences. Whether they explain validity or merely mimic it is still argued, and the honest summary is that expressivism survived the objection at the cost of enormous complexity.
Example. A friend says: "Morality is just evolution. We help our relatives because it spread our genes, and that is all there is to it." What is right and what is wrong in this?
The evolutionary claim is largely correct and by itself establishes nothing, since it is an explanation of why we make moral judgements rather than an evaluation of them. Taken as an argument it commits the genetic fallacy: showing how a belief arose does not show it false, or arithmetic would be in trouble too, since counting also has a selective explanation. The serious version is Street's, which does not say the beliefs are false but that a realist has no story about why selection would have tracked truth, so the beliefs are unreliable on realist assumptions. That is an argument about justification rather than origin, and it is much harder to answer. Notice also that the friend's claim, taken at face value, licenses nothing about how to behave now, since we routinely act against inclusive fitness and do not regard that as an error.
Now you. Does the following argument work? "There are no objective moral facts. So nothing is really wrong. So I may do as I please."
Answer
The first step is a position, the second follows on an error theory, and the third does not follow at all. "I may do as I please" is itself a moral claim, a permission, and on an error theory it is false along with everything else. The error theorist's actual conclusion is that no one is permitted anything either, which is not a licence but a void. What people mean when they draw the inference is usually practical rather than logical: if nothing is objectively wrong, no one has standing to stop me. That is false too, since what stops people is other people's attitudes and institutions, whose existence no metaethics touches. Mackie himself, having concluded that moral facts do not exist, wrote a substantial second half of his book on what morality should be invented to look like.
Why the metaethics does not settle anything
Now the point of the lesson. Suppose you accept, having read the arguments, that expressivism is true and there are no moral facts. What changes about the argument over euthanasia you have to have tomorrow?
Almost nothing. You still have attitudes, and you still find some of them inconsistent with others, and consistency is exactly what moral argument trades on. You still discover that your objection to euthanasia rests on a principle about killing that you do not apply to switching off ventilators. You still find that the principle you would need to defend your position condemns things you approve of. Every move made in the remaining lessons of this course is available to you, because every one of them works by exposing tension between commitments, and tension between attitudes is as real as tension between beliefs.
This is the point of quasi-realism, and it is why the position was built. Blackburn's programme was to earn the right to talk exactly like a realist, to say that some moral views are better than others, that we can be mistaken, that torture would still be wrong if everyone approved of it, all from expressivist materials. To the extent it succeeds, the practical difference between the two positions is nil.
The one thing that does change is the character of a final standoff. If two people have run out of shared premises and disagree at the foundations, the realist thinks one of them is wrong about something, and the expressivist thinks they simply differ. That matters for what you say at the end of an argument, and not for anything you say during it. Since almost no real disagreement gets that far, the metaethics has less practical purchase than its prominence suggests.
Example. Which position does each claim belong to? (a) "Nothing is wrong, though we should keep talking as if things were." (b) "Wrong means forbidden here." (c) "Saying it is wrong is not saying anything true or false." (d) "It would be wrong even if everyone approved."
(a) is error theory in its fictionalist form, keeping moral talk as a useful pretence. (b) is cultural relativism, a species of subjectivism. (c) is expressivism, denying that moral sentences are truth-apt. (d) is what realism straightforwardly asserts, and it is also what quasi-realism claims to be able to say, which is exactly why sorting positions by their slogans is unreliable and you have to ask what makes the slogan true.
Now you. Your interlocutor is an error theorist. What can you still say to them about factory farming?
Answer
Everything except that it is objectively wrong. You can point out that they object to gratuitous cruelty to dogs and ask what property of a pig makes the difference. You can show that the principle they invoke to permit the practice, that a being's suffering matters only if it is human, is one they reject in other applications. You can present the conditions and see whether their reaction survives the description. None of these moves requires the existence of a moral fact; they require only that your interlocutor prefer coherence to incoherence, which almost everyone does. That is the working assumption of the rest of this course, and it is why the next lesson is about method rather than about metaphysics.
Where this leaves the reader
The state of professional opinion is worth knowing without being deferred to. The 2020 PhilPapers survey of academic philosophers found that just under two thirds accepted or leaned towards moral realism, with about a quarter on the anti-realist side, which is a majority rather than a consensus.
The safe conclusion is that the metaethical question is open, and that its being open does not stop anything. Every position on the list generates the same next task: work out which acts to defend, which principles support them, and what to say when the principles collide.
That task has a method, and the method is teachable. The next lesson sets it out: how a moral argument is put in a form where it can be attacked, what a counterexample does to a principle, what thought experiments are good for, and what it means for a set of moral judgements to be in good order.