See transcript below.

Science can explain many things in the world.  Biology can explain why butterflies are attracted to flowers or how cells generate energy.  Psychology explains why interpersonal conflict occurs.  Physics can explain how light refracts, forming a rainbow.  But there is one thing many philosophers believe science cannot explain.  And that’s the nature of morality.

Many social scientists describe the moral codes of different cultures or the psychology behind moral actions.  But we’re pursuing a different issue.  Metaethics is interested in explaining why some moral claims are true and others false.

Consider how we might determine if this claim is true: “The average human lifespan in 2016 was 78 years.”  We might conclude this after observing natural facts about lifespan and researching the data.  We would use science.  Now consider how we’d figure out if these claims were true: “It’s wrong to arbitrarily kill someone,” or “We ought not arbitrarily kill someone.”

Many philosophers believe that no amount of observing and analyzing natural facts would give us an answer.  Rather, we need to engage in a philosophical investigation of the nature of morality.  The task is to figure out what makes moral claims such as these true.

We’ll start with cultural relativism.  For the cultural relativist, a moral claim is true or false depending on whether the culture endorses the claim.  Furthermore, moral norms are understood as social conventions, cultural constructs.

We’ll then examine a theory call moral subjectivism, which claims that moral statements are true relative to an individual.  The moral subjectivist, like the cultural relativist, believes that morality is not objective.  Moral Realism, on the other hand, claims that morality is objective and that some moral claims are true regardless of whether an individual or a culture endorses them.

Next, we’ll look at metaethical constructivism.  You can think of this theory as sort of an intermediary between relativism and realism.  Metaethical constructivism holds that what makes moral claims true is that a group of people deliberating in an idealized, hypothetical forum would agree upon them.

Finally, we’ll examine a family of theories called moral nihilism, which claim that moral truth doesn’t exist.  For the nihilist, there are no moral facts in the world that would make moral claims true.  Morality is an illusion.

Metaethics is also concerned with moral norms. There are many kinds of norms that govern our behavior and give us reason to perform certain actions.  Prudential norms tell us how to act in our best interest.  They’re oriented around what’s valuable to each of us.  Moral norms and prudential norms may not always recommend the same action.  For example, it may be in your best interest to cheat on your taxes if you can get away with it, even though it’s morally wrong.

Norms of etiquette and custom are conventional norms that dictate which behavior is polite or impolite, offensive or inoffensive.  These norms can also conflict with moral norms.  For example, while it may be impolite to shout in an art gallery, it’s the morally right thing to do to warn others of a fire.  Laws are also a kind of norm, which indicate what actions are punishable by the state.

So, what sets moral norms apart from other kinds of norms?  First, moral norms usually provide us with stronger reasons for action.  For example, norms of prudence would recommend that you shouldn’t help someone who is injured while you’re walking to the office to make an important meeting, since, if you stop to help him, you will be late to the meeting and your uncompromising boss might fire you.  Even though it might not be in your best interest to do so, morality requires that we stop to help the injured person.

Second, criticizing someone for flouting a moral norm is usually much more significant than criticizing someone for flouting a norm of etiquette or prudence.  For example, criticizing someone for breaking a promise may involve strong feelings of indignation or resentment.  It may even involve breaking relations with that person.  However, criticizing someone for chewing with his mouth open may merely involve a wag of the finger and it would surely be unreasonable to feel resentful toward him.

To sum up: we examined how science is ill-equipped to explain the nature of morality, how a variety of metaethical theories grapple with moral truth, and why moral norms stand apart from other kinds of practical norms.