Free will

Desirism requires determinism.
It is often thought that morality requires a "free will" - an ability to act in ways other than the determined laws of physics would cause one to act.
To say that an agent ought to have done something else implies that he could have done something else. Which means that if it is not the case that he could have done something else, then it is not the case that he ought to have done something else.
Determinism - which holds that our behavior is determined by prior causes in the same way that the motion of an object through space is determined by prior causes - says that a person could not have acted differently. The laws of physics determine the motion of every electron, proton, and neutron on our body, meaning that they determine the motion of the body itself. It is never the case that an agent could have overthrown these laws of physics and done something else.
The problem of determinism persists, even where we allow that some events in nature are truly random. Random events are not determined - they are governed by chance. However, there is still a clear distinction between randomness and choice. If it were the case that one's arm would randomly fly up into the air, or if a person were to randomly utter certain words, this is not the same as saying that he freely wills to raise her arm or that she chooses her words.
Because there is no element of genuine choice in human action, there is no sense that a person "could have done otherwise" in the relevant sense. This means that it is never the case that an agent "should have done otherwise" in the relevant sense. Morality is built on a fiction - a myth - and has no place in the real world where the motion of matter through space is determined by the laws of physics with some possibility of randomness.
Desirism and Determinism
Desirism tackles the free will problem by arguing that the argument above makes a mistake in how it understands "could have done otherwise."
Desirism requires determinism. With desirism, if free will did exist - if humans actually had the capacity to overthrow the laws of physics and act in violation of those laws - this would introduce a complication that the theory could not handle.
Desirism requires it to be the case that our actions are caused by our beliefs, desires, habits, and the like. It focuses on desires precisely because we have no capacity to act in ways independent of our desires. It further requires that desires determine ends or goals and that agents act so as to realize those aims or goals. It requires that some desires are malleable - meaning that interaction with the external world can create, strengthen, weaken, or exterminate those desires. It requires that the types of experiences capable of molding desires includes praise, condemnation, reward, and punishment. It requires that those who use these tools have the ability to predict, at least roughly, what their effects will be on the desires - and through them on the actions - of other agents. It says that, with this ability to make these predictions, people have reason to use the tools of praise and condemnation to promote some desires and inhibit others - a practice that would be severely complicated if free will actually existed.
These facts make it possible for one person to influence the actions of another by influencing the causes of those action, and to influence the causes of action by using tools that themselves have an effect on the causes of action. Those tools - praise, condemnation, reward, and punishment - act on the reward center of the brain to alter the desires ofother agents. If we throw free will into the soup, the system becomes a hopeless muddle. It is best to leave it out - unless somebody can provide real evidence that it is real.
Praise, Condemnation, Free Will, and Malleable Desires
The question remains, "How can you condemn somebody by saying he should have done something else when he could not have done something else?"
However, defenders of desirism ask a counter-question: "How is it the case that praise and condemnation are a legitimate response to an act of free will?"
Perhaps a legitimate response to a good act is to spin clockwise and to a bad act is to spin counter-clockwise. It makes no sense - there is nothing in a good act that actually implies that agents should spin clockwise, or in a bad act that implies that an agent should respond by spinning counter-clockwise. However, it is also the case that there is no valid inference from the fact that a good act was a product of free will that the agent should be praised, or from the fact that a bad act was a product of free will that a response of condemnation makes sense..
Desirism links good and bad actions with praise and condemnation through the determined effects of praise and condemnation on desires - the causes of good and bad action. It calls for the condemnation of bad actions because condemnation will promote aversions and inhibit desires that tend to bring about action types that are like the action being condemned. At the same time, it calls for the praise of good actions because praise tends to strengthen desires that cause and weaken inhibitions that prevent actions like the act that is being praised. There is a specific reason for responding to bad acts with condemnation and good acts with praise - a reason that makes sense in a world of cause and effect.
Could Have Done Otherwise
On the issue that it is wrong to condemn somebody unless they could have done otherwise, desirism uses the compatibalist account of "could have done otherwise". This account reduces "could have done otherwise" to "would have done otherwise if he had wanted to," and "could have wanted to."
"Could have wanted to," in turn, refers to the fact that the wants that are relevant in making a moral evaluation are malleable desires - desires that can be molded through acts such as praise and condemnation. "Ought" implies "can" comes from the fact that it only makes sense to apply the tools for molding desires where desires can actually be molded, and where different desires would produce different results. It makes no sense to apply these tools in cases where actions are not the product of malleable desires.
The Illusion of Choice
A final concern on the issue of free will is the question of how to deal with the illusion of choice. When an agent is deciding whether to take the money out of a co-worker's desk drawer, he seems to actually have a choice. When he is trying to figure out what to order on a menu, or where to go on vacation, he seems to be making a choice among possible futures, each of which is genuinely possible.
However, determinism seems to imply there is no actual choice - choice is, at best, an illusion. The agent either will or will not take the money. What the agent will have for dinner and where he will go on vacation are already written into the world, and we merely need to sit back and see the results.
Desirism replies by denying that choice is an illusion. People make genuine choices. However, the process that they go through in making a choice does not require free will.
Computers have shown us how choice is possible in a determined world.
A chess-playing computer goes thorough a set of possible moves, measuring the outcome of each option, and then deciding on the outcome with the highest value. It evaluates the different moves available to it as if each move were a genuine possibility. It evaluates KP2-KP4 and determines the value of the resulting states as if it actually has the option of choosing KP2-KP4, and it evaluates QP2-QP4 and determines the value of its resulting states as if this is a real option. Then, it picks the option that realizes the highest value.
We can ask a question about the machine, "Why is it evaluating these options as if they are real options when all outcomes are determined?"
The answer is that it is because the outcome is determined by a system that examines each option and determines the value of resulting states. When the computer examines KP2-KP4, this is a possible move in any sense that matters. The sense that matters is the sense that, if the computer determines that this move would produce the result with the highest value, then the computer will make this move. Or, in other words, the computer could have moved KP2-KP4 if it had wanted to.
This does not prove that computers are moral agents. However, it does demonstrate how choice is possible in a determined system.
It also provides us with an account of what would be required for a type of machine morality. To create a machine morality, one needs a system by which one machine can alter the values that another machine attaches to particular states by influencing the environment in which that other machine operates. This, in turn, requires that the values that other machines attach to particular ends can be changed in ways the first machine can predict.
Conclusion
Morality is not only possible in the absence of free will, morality requires determinism. It requires that actions be caused by beliefs, desires, and other mental states. It requires that some desires are malleable in the sense that interactions with the environment will influence their strength and even their existence. It requires that agents can mold the desires of other agents by molding the relevant interactions with the environment. It requires that the desires of agents give them reason to influence the relevant interactions between other agents and the environment - to provide praise and condemnation, rewards and punishment.
Free will, if it existed, would only add unwanted complications to the moral system.