I often say, "Desires are the only reasons for action that exist."
However, we should be clear. My desires are reasons that exist for my actions. Your desires are reasons that exist for your actions. My desires are not reasons that exist for your actions, nor are your desires reasons that exist for my actions.
As it turns out, if everybody's desire was a reason for everybody else’s action, we would have no need for (criminal) law or morality. Everybody would automatically consider everybody else’s interests. It is because reasons for action cannot cross personal boundaries that we have a need for (criminal) law and morality.
So, I have desires that would be fulfilled by performing action A.
Action A has an effect that can be presented in the purely descriptive statement D.
You have a particularly strong aversion to D (e.g., D is a state in which you are in excruciating pain or one in which your child is made to suffer permanent and severe harm).
You tell me that I should not do A.
I ask, "Why?"
You answer, "Because A will bring about D"
I answer, "So?"
If you want a purely descriptive statement to have any relevance to my decision as to which action to perform, then it must either be the case that I have a reason not to perform D (D will thwart some desire of mine), or D will bring about E and I have a reason to not bring about E.
(We need to add caveats in that my actions will also depend on my beliefs. However, for the sake of simplicity, let us assume only true beliefs for now.)
The fact that D is something that you have reason to avoid is not, in itself, a reason for me not to bring about D.
However, your aversion to D (e.g., your aversion to pain or your aversion to your child suffering permanent harm) is a reason for you to give me a reason not to bring about D.
You have two ways to give me a reason not to do A.
Method 1: You say to me, "You also have a desire that Q. If you perform A, then I will thwart your desire that Q."
Method 1 is the way of criminal law.
"Thwart your desire that Q" involves such things as fines, imprisonment, maiming, and execution. All of these tend to universally thwart desires of those who are subject to them. Consequently, all of these give others a reason for action to not perform the act that would result in punishment.
Method 2: You act in ways that alter my desires. You either reduce or eliminate the desires that would be fulfilled by my doing A, you give me an aversion to doing A itself, or you give me an aversion to some effect of A (such as an aversion to causing others great pain or to do permanent harm to children).
Method 2 is the way of morality.
If you can give me an aversion to causing others great pain, then I can no longer respond to your statement, “Your action will cause me great pain," by saying "So?" If I have an aversion to causing others great pain, your statement that A will cause you great pain, added to my aversion not to cause others great pain, means that I have a reason not to do A.
If that aversion to causing others pain is made sufficiently strong, then I can be given more of a reason not to do A then I have for doing A.
Method 2 has certain advantages over Method 1.
Threats to thwart the desires of others depend (a) on having enough power to carry out the threat, and (b) on the ability to acquire enough information to direct the threat effectively. Method 1 is not effective against those who are too powerful to be punished, or on those who have an opportunity to perform A without getting caught.
However, Method 2 works even on those in power, and even on those who can act without getting caught.
If I am an extremely powerful individual, then I am going to put that power to work fulfilling the most and strongest of my desires. If those desires include an aversion to causing others pain, then that is one of the desires that I am going to use my power to fulfill. My power would be no threat to you.
If I have a strong aversion to causing your pain then it does not matter whether I can cause you pain in secret – and get away with it. People do not do things that they have reasons-for-action not to do even in secret. Nobody has to worry about the person sneaking food that he hates when nobody is watching him, or of doing great harm to somebody that he loves when he can get away with it. He does not want to do these things, so he will not do them, even in secret.
So, I have desires that would motivate me to perform action A.
You respond that I should not do A.
I ask, "Why?"
You say, "Because it is wrong."
I say, "So?"
Assuming that I have no aversion to doing what is wrong, you are stuck. The claim that something is wrong is not necessarily a reason for me not do it.
However, the claim, "Because it is wrong," is a flag to everybody that people generally have many and strong “reasons for action that exist” to bring Method 1 and Method 2 to bear against those who have desires that would motivate them to perform Action A. It is not a reason for action for me not to do A, but it claims that there are reasons for action for people generally to act in ways to give me a reason not to do A.
There is one more point that I would like to make. Even if I have no aversion to causing people pain, I have “reasons for action” to promote an overall aversion to causing pain. Even if I have no aversion to taking the property of others without their consent, I have "reasons for action" for promoting an overall aversion to taking property without consent. And even if I have no aversion to wantonly killing others, I have "reasons for action" to promote an overall aversion to killing others.
Which means if a person answers, "So?" to the statement, "X is wrong," one possible response is, "How would you like to live in a world where people freely go about causing everybody pain?" Or, "What if everybody did that?"
So, I might not have an aversion to causing others pain, and I might not have an aversion to doing that which is wrong. However, I cannot simply say, "So?" to the statement "X is wrong," because this typically means that even I have many and strong reasons to promote an aversion to people doing X.