Survey of moral realism and irrealism

Note: philosophers use "ethics" and "morality" as the same word.

The moral realism vs irrealism contrast is the most important contrast in metaethics, the study about what ethics is, such as whether morality is objective, subjective, emotion-based, rationality-based, changeable over time, etc.

Realism says morality is real in some sense, while irrealism says it isn't.

Infinity

For an example of such a contrast, we can consider the positions on infinity in mathematics. Realism would simply say that infinity exists, is an actual mathematical object, and there's a symbol of it, ω. Irrealism would claim that ω is just a convenient fiction, and merely stands for the endlessness of natural numbers.

There are very fine-grained notions of realism vs irrealism in the study of infinity, too, used by expert set theorists. Most set theorists consider the first few infinities unproblematic, but as they climb up the ladder of infinities, they encounter more and more abstract infinities. They often draw a line somewhere and proclaim infinities beneath some line real and above unreal.

Figure from the cover of The Higher Infinities (2002), Akihito Kanamori

Gender

As another example, people tend to be natural realists about gender, while sociologists tend to be more nuanced. They think gender is only socially real, a social construct based on biological sex (which is naturally real). In a bit more detail, gender is usually thought of as a big bag of personality traits (calculating), social behavior norms (don't flirt with males), personal preferences (like using make-up), etc.

Sociologists point out how this two big bags of stuffs is not a "natural" way to divide the natural diversity of humans, and that how these bags are very different in different societies, or in the same society over time, and how in some cultures there are extra bags (hijra, eunuch, nonbinary, etc).

Existence

Nihilism is "irrealism of something". There are many kinds of nihilism and I'll list some.

• Metaphysical nihilism usually says "it's possible for a universe to have nothing in it" and more rarely says "this universe has nothing in it". 
• Buddhism is nihilistic about the existence of self. 
• Existentialism is nihilistic about the objective meaning of life. 
• Absurdism is nihilistic about any meaning of life, objective or subjective.
• Mereological nihilism says that objects with proper parts do not exist. For example, since humans are made of atoms, humans don't exist, since otherwise atoms would be proper parts of humans. Atoms don't exist either, since they'd be made of electrons and stuff, etc.
• Materialism is nihilistic about spirits and minds.
• Moral nihilism is moral irrealism, which we'll discuss today.

Varieties of Moral Realism vs Irrealism

There are many ways to contrast these, just like there are many nihilisms. We'll follow Moral Anti-Realism (2015).

To hold a realist position with respect to X is to hold that X exists in a mind-independent manner. Moral anti-realism denies that moral properties—or facts, objects, relations, events, etc—exist mind-independently.

 There are multiple ways to do that:

1. Deny that moral properties exist at all.
2. Accept that they do exist but that existence is mind-dependent ("subjective"). 

There are three main theories: noncognitivism, error theory, non-objectivism.

Moral noncognitivism

Moral noncognitivism holds that our moral judgments are not supposed to be talking about truth, but should be understood as expressing attitudes/wishes/feelings/other things. For example, if I say, "Stealing is wrong." I'm really saying "I demand everyone to not steal." This is similar to how saying "Close the door now!" is not saying "The door will be closed in a minute."

In contrast, moral cognitivism says that our moral judgments are talking about truth.

Moral error theory

Moral error theory holds that our moral judgments are talking about truth, but we can't ever get to know them, and we are always in error when we think we got any moral knowledge. In contrast, moral... correctness? theory holds that we can have some moral knowledge.

It is the attitude that sensible people take toward phlogiston, astrology, the Loch Ness monster... An error theorist doesn't believe in such things; she takes talk of such things to be a load of bunk. The moral error theorist doesn't believe in such things as moral obligation, moral value, moral desert, moral virtue, and moral permission; she takes talk of such things to be bunk. 

As an example, we are all error theorists when it comes to phlogistonics:

When a 17th-century chemist said “Phlogiston resides in combustible materials” he was making an assertion; i.e., nobody is a noncognitivist about 17th-century phlogiston discourse. But we think that such assertions were systematically untrue, since there is no phlogiston.

A moral error theorist can still talk about morality, though. She could still honestly say things like "People believe in morality." And she could lie and say, "Stealing is bad. Lying is bad too." Though she probably wouldn't find lying morally bad, since she doesn't believe it. This is why if a professed moral error theorist employs morality talk, it can be consistent.

Indeed, there's a whole area called fictionalism that basically says "This kind of talking (morality, or some other thing) is untenable when we think about it philosophically, but it's fine to still talk like that outside of the philosophy circle like a convenient fiction."

Fictionalism is commonly accepted in pretend-plays, like "The Floor is Lava".

Practically, moral error theorists tend to be still pretty riled up about punishing immoral actions and rewarding moral actions like normal people, but without appealing to morality like normal people. The error theorist usually rejects morality as requiring some ingredient X (be it free will, universal behavior guidelines for all homo sapiens, or whatever) that our universe simply cannot have. Then they recover the practical consequences of morality by justifying it with something else that does not require ingredient X.

Such a practical moral error theory faces two kinds of challenges.

The challenger may acknowledge that X is problematic, but deny that this attribute is an essential component of morality; a normative framework stripped of X will still count as a morality. Alternatively, the opponent may accept that X is a non-negotiable component of anything deserving the name “morality,” but deny that it really is problematic. 

This ingredient X can be annoyingly vague:

Moral properties have a “to-be-pursuedness” to them (Mackie 1977: 40), moral facts would require that “the universe takes sides” (Burgess [1978] 2007), moral believers are committed to “demands as real as trees and as authoritative as orders from headquarters” (Garner 1994: 61), and so on.

Moral non-objectivism (strictly speaking requires a tiny amount of moral realism)

To deny both noncognitivism and the moral error theory suffices to make one a minimal moral realist. Traditionally, however, moral realism has required the denial of a further thesis: the mind-dependence of morality.

Moral non-objectivism holds that our moral judgments are talking about truth, and we can get to know some, but like colors, moral truths are only subjective, not objective.

There is nothing either good or bad, but thinking makes it so. -- Hamlet

In contrast, moral objectivism holds that there are objective moral truths that don't depend on human minds.

Although, this distinction is kind of weird, since our thoughts are objective things in an objective world. For example, consider the claim

The color red doesn't exist, since it's not a natural property, and only constructed in our minds.

Then we can object:

The color red exists as an electric firing pattern (or some other physical phenomenon).

Similarly, consider one possible way to be non-objective about morality:

Morality isn't objective, since it's not a natural property, and what's right or wrong is just what we think is right or wrong.

Then we can object:

Morality is objective. It's a set of rules that social animals follow in interactions with other animals, especially of their own kind.

Motivation for moral realism vs irrealism

Both positions are difficult. Like the contrast between free will vs determinism, or dualism vs materialism, one position is more intuitive, the other more scientific/naturalistic.

On the one hoof, moral realists face a cluster of explanatory challenges concerning the nature of moral facts (how they relate to naturalistic facts, how we have access to them, why they have practical importance). On the other hoof, it is widely assumed that intuitions strongly favor the moral realist.

(Meta)intuitively, moral realism is intuitive, and irrealism is unintuitive and only people who thinks hard about morality could doubt the reality of morality, right? Not so fast.

Do commonsense intuitions really favor moral realism? ... the fact that we are unwilling to defer to experts when forming moral views seems to count against realism (McGrath 2008). Similarly, the fact that we do not expect a person necessarily to accept others' reasons for their moral views seems to reveal anti-realistic tendencies.

Though in my guess, it's just the Dunning–Kruger effect in ethics:

1. The experts aren't that reliable, because they couldn't reach a consensus on many things (true).
2. I'm better than most people on thinking through ethics problems (Dunning–Kruger)
3. Thus, I should not give up my ethics beliefs just because the experts said so.

Also, even if people are intuitively moral realists, intuition isn't what physics is based on, so perhaps it's not useful to base ethics on intuition. And popular intuition doesn't help in finding religious truth:

... wherever one stands, one must credit millions or billions of humans with radically false fundamental religious beliefs. Knowing that humans can be massively and systematically mistaken encourages the moral anti-realist to deny that popular opinion in favor of moral realism would constitute a burden he needs to overcome.

As such, the question of who has the burden of proof is far from clear. And there's plenty to disagree and argue interminably about for philosophers.

Also, relativism isn't irrealism

Relativism holds that moral claims contain an essential indexical element, such that the truth of any such claim requires relativization to some individual or group.

For example, a moral relativist could say that what's moral is just what's socially acceptable. Then it's perfectly possible for some people to claim slavery is good, while others to say slavery is bad, and both would be right.

A moral irrealist would be saying that they are both talking about something that's not right or wrong.

Moral relativism often looks very similar to moral non-objectivism, though, and I wouldn't bother making the distinction. It does look quite different from moral noncognitivism and error theory.