Why Should I Accept Your Definition Of Morality

"Why should I accept your definition of morality?"

This is a common question.

The answer to this question is, "I cannot give you an objective proof that the definition I provide is the one and only absolutely correct definition or that you make a mistake using a different definition."

At this point, some critics are quick to respond, "That proves it! All morality is subjective. One person's morality is as good as any other. Furthermore, my morality is unassailable. Nobody can say anything against it that is not itself an absolutely arbitrary, subjective claim."

This last response is unsound.

Consider the following counter-example.

An astronomer says that a planet is an object that is too small to create energy through nuclear fusion, that is round by its own gravity and, when orbiting a star, has swept its orbit of all similar debris. Furthermore, he says, Pluto is not a planet.

An astronomy student asks, "Why should I accept your definition of a planet?"

Clearly the definition violates the historic use of the term. Pluto has been called a planet since its discovery.

The astronomer says, "It is convenient to have terms refer to things that are alike. Pluto is more like a  Kuiper Belt Object than like any planet. Therefore, we are going to stop calling it a planet and start calling it a Kuiper Belt Object. If this upsets too many people, we will call it a dwarf planet.  We can give you no argument proving that this is the one and only absolutely accurate definition of "planet."  it isn't. It's just the definition we agreed to use. All I can do is urge you to go along with the decision.

Indeed, if somebody insists on continuing to use the old definition of "planet," they cannot be proved to be incorrect. They may be called stubborn. People may complain that their refusal to go along with the crowd sews confusion. However, they cannot be charged with making a factual error.

At this point, the student says, "That proves it! All of astronomy is subjective. Whether Pluto is a planet depends on what definition of planet you use, and there is no objectively correct definition of the term. There is nothing to convince others to adopt the definition except coaxing, coercion, and bullying."

Everything the student says about the definition of "planet" is true. However, the inference from these facts to the conclusion that all of astronomy is subjective is invalid. Astronomy remains a hard science - a paradigm example of objectivity - in spite of these facts. A teacher can ask a test question, "How many known planets are there in the solar system" and count as wrong anybody who gives an answer other than "8".

The same is true of morality.

Desirism is called a moral theory because it deals with a wide range of topics commonly associated with morality. These include the use of praise, condemnation, reward, and punishment; concepts such as "excuse" and "supererogatory action", why it makes sense to ask, "What if everybody did that?" and the like. These are concepts long associated with the term "morality" by convention.

People readily identify a discussion of these topics as "morality" just as they recognize that a discussion of planets, stars, moons, galaxies, comets, and nebulae as making up a field called  "astronomy".

However, in the same way that the astronomer cannot prove that a particular definition of "planet" is the one and only correct definition, nobody can prove that the definition of "morality" is the one and only correct definition. 

Astronomers recognize that different definitions of the word "planet" do not change Pluto's size, orbit, chemical composition, number of moons, or any other fact.

Similarly, different definitions of the word "morality" do not change the reasons for action that exist, the role of beliefs and desires in forming intentional actions, the fact that some desires are malleable, that praise, condemnation, reward, and punishment can mold malleable desires, that there are malleable desires that people generally have reason to promote, or that there are areas where people generally have reason to see people adopt diverse interests. These facts are facts regardless of the name we give them.

You cannot magically conjure reasons for action that exist just by changing definitions any more than you can alter Pluto's size by changing the definition of "planet".

A person can choose a different definition if they want. Whether or not they have acted in a way that people generally have many and strong reasons to condemn will not be affected.