Inspired by this tweet.
There are a lot of proposals for what makes an action morally good or bad. These normative ethical theories can take various forms, but the most plausible are types of utilitarianism. This is because utilitarianism allows agents that follow it to behave rationally.
The von Neumann-Morgenstern utility theorem shows that any agent that does not behave as if it has a utility function can have arbitrary amounts of resources pumped from it. This is a huge problem for any normative ethical theory that does not allow agents to behave this way. “Moral” agents would be unable to reliably perform obviously good acts like life-saving surgery because another agent could come along at any time and trick them into giving up the surgery equipment or other necessary resources. Since this is intuitively not how morality works, morality must be some form of utilitarianism.
Does this mean that other theories are doomed? Not necessarily. It turns out that a lot of them can be easily converted into utilitarianism. For example, virtue ethics can be converted simply by holding “being as utilitarian as possible” as the only virtue worth having. Deontology can be similarly converted by making “act as a utilitarian would” the only rule.
Other theories are much harder to convert. Contractarianism requires that everyone agree to a specific social contract, so if this contract is not already equivalent to some form of utilitarianism, then it would have to be “renegotiated”. This would be a daunting task, as it would require every single member of society to consent to a new utilitarian contract. Luckily, making a few reasonable assumptions allows us to show that we must have a social welfare function that consists of a weighted sum of individual utility functions. Since this is itself a utility function, this ensures that the social contract is indeed utilitarian.
These are not the only normative ethical theories in existence. There are plenty of others which should be examined for possible equivalences to utilitarianism and discarded if they are not found to have any. Of course, it’s also acceptable to simply apply utilitarianism directly rather than through some other theory, but having different ways to frame the matter is useful for making things as intuitive as possible. This might ultimately reach a point where all normative ethical theories are either unified in this way or dismissed as irrelevant, but until then, we must make do with the equivalences we have.