Thomas Hobbes is credited with providing one of the cleanest accounts of common moral relativism. It is also an account in which we can clearly see the mistake of moral relativism.
Whatsoever is the object of any man’s appetite or desire; that is it which he for his part calleth good; and the object of his hate, and aversion, evil; and of his contempt, vile and inconsiderable. For these words of good, evil, and contemptible, are ever used with relation to the person that useth them. (Thomas Hobbes, Leviathon, Book 1, Chapter 6)The statement starts off well enough.
It is true that "appetite or desire" is the source of all goodness, while "aversion" is the source of all that is bad. Here, I am referring to things that are good or bad for their own sake - and not because they serve as a means to something that is either good or bad. There is nothing simply and absolutely good or bad - good or bad because of its intrinsic properties such that it would retain its value in the absence of beings with appetites, desires, and aversion - in the universe.
Technically, there is no need to limit this to "man", either in the gendered sense, or in the sense that means "human being". Animals, too, have desires and aversions. What is good or bad for the animal is that which the animal desires or that to which an animal has an aversion. An animal has as much reason to avoid its own pain as any human does, and would use those terms if they were able to. I do not know if Hobbes would embrace this view with respect to animals - he may have thought of animals as mere machines incapable of actually experiencing pain or having desires. However, we can certainly know that this is true of animals as well.
This view is consistent with Bernard Williams' thesis regarding reasons for action.
There is a reason for A to φ iff there is some desire the satisfaction of which will be served by his φ-ing.It is seldom specified, but it should be, that an aversion is simply a desire that something not be the case. Thus, there is a reason for A to not-φ iff there is some aversion the avoidance of which will be served by his not-φ-ing.
However, after this very promising start, and with no explanation, Hobbes, like a great many "moral relativists", goes off the rail.
Why is it that the terms "good" and "evil" can only be used with "with relation to the person that useth them?"
The idea that we can relate objects of evaluation only to our own desires and aversions is as bizarre as claiming that we can only describe the location of something relative to ourselves, and never talk about their location relative to somebody else. Where we may otherwise describe the location of Cairo relative to Egypt, and the location of Egypt relative to Africa, this bizarre limitation on language would prohibit us from reporting the location of Cairo or Egypt or Africa in any terms other than its direction and distance away from the speaker.
No sensible person would impose such a requirement.
In fact, there are very few claims that are so easily shown to be false. It is true that all value-laden term relate the object of evaluation to desires (at least insofar as the terms are a part of a true value claim). However, it is not the case that we can or do relate objects of evaluation only to our own desires. We have the capacity to know that other people exist. We have the capacity to know that other desires exist. We have the capacity to recognize the relationships that exist between objects of evaluation and those desires. We have the habit of reporting those relationships using value-laden terms.
For example, I say, "I want to buy a good present for my dad for Father's Day."
The term here does not refer to my own desires. My want to buy a good present relates to my desires, of course. However, the "good present" I am looking for - that which would make the present a good present or a poor present - would be one that would fulfill my dad's desires, not my own. Everybody who would hear such a statement would understand whose desires are relevant when it comes to evaluating possible presents.
The same applies to a case where a parent tells a young child, "Eat your vegetables. They're good for you."
The phrase "good for you" - as well as "good for us", "good for them", "good for the community" all relate objects of evaluation to a set of desires belonging to the person or group that the object of evaluation is said to be good for.
Indeed, our ability to navigate the world in which we live depends on our ability to predict the behavior of other individuals. We predict the behavior of other individuals by recognizing the value of particular objects of evaluation relative to their desires, under the assumption that they will act so as to fulfill the most and strongest of those desires. We try to please our spouses, satisfy our bosses, and avoid conflicts with strangers, by determining how the states of affairs we create relate to desires and aversions that are not our own.
It is true that when we relate an object of evaluation to desires not our own, that we are not necessarily motivated to realize that which would be called "good" or avoid that which is called "bad" in this sense. The fact that a state of affairs would fulfill a desire that my neighbor has means nothing to me unless I have a desire that my neighbor's desire be fulfilled. I may hate my neighbor and wish for him to have a bad time - perhaps hoping that he has a particularly bad vacation. The value of the vacation here, as above, is determined by measuring the vacation against my neighbor's desires, not my own.
Once we recognize that evaluative language can relate objects of evaluation to desires and aversions other than those of the person who is using the terms, we recognize that we may have an alternative way of accounting for moral terms. Moral terms do not relate objects of evaluation - be they character traits or actions - to the desires of the speaker. They relate those traits to the desires and aversions of people generally - including the speaker. One way of understanding moral terms is as evaluations of character traits or types of action that are "good for us".
Technically, desirism argues that the ultimate objects of evaluation are malleable desires - specifically, desires that we can mold using the social tools of reward, punishment, praise, and condemnation. The desires that are relevant in making these evaluations are not those of the speaker, but those that are found within the community. In other words, desirism holds that malleable desires themselves are to be evaluated according to the standard of whether they are "good for us" or "bad for us".
And it is worth noting that goodness or badness in this sense is not grounded on an account of what people BELIEVE is "good for us", or "bad for us". It is concerned with what is "good for us" or "bad for us" in fact. Because there is a fact of the matter, this is something that people can debate - and something about which some people - potentially, even, that which everybody in a community - can be just plain objectively wrong.
This is how, even though morality is relative (depending on "our" desires and aversions), it is also, at the same time, objective, real, and independent of the beliefs that dominate any community.
Once we recognize that relativism is compatible with moral realism and objective moral facts independent of belief, we recognize that many of the debates people are having on the nature of moral value - particularly debates about whether morality is relative or objective - are a waste of time. It is like arguing over whether a red car is red, or if it is a car, with each combatant making the mistake of thinking that which is red cannot be a car and that which is a car cannot be red. In fact, something can be both. And, in fact, morality can be relative (depending on "our" desires) and real and objective at the same time.
Add to this, two more facts:
Fact 1: People CAN cause others to be concerned about what is "good for us" to be concerned about - using (not reason, but) praise, condemnation, reward, and punishment.
Fact 2: People have reasons to cause others to be concerned abut what is "good for us" since they are the "us" that others would then be concerned with.
Now, you have everything you need for an moral system with objectively true and false moral claims that are independent of what anybody believes is moral or immoral - a real-world morality.