Many of the claims of desirism find their origin in the writings of the 17th century Scotish philosopher, David Hume
Sentiments Are the Object of Moral Evaluation
For example, desirism holds that desires are the primary object of moral evaluations. Actions are to be evaluated on the derived status of whether they are or are not those actions that a person with good desires would perform.
Furthermore, good desires are determined by whether the desire tends to objectively satisfy other desires, either directly or indirectly.
In David Hume’s book Enquiry Concerning the Principles of Morals Section IX, Hume wrote:
It may justly appear surprising that any man in so late an age, should find it requisite to prove, by elaborate reasoning, that Personal Merit consists altogether in the possession of mental qualities, useful or agreeable to the person himself or to others.
The mental qualities that Hume refers to are character traits – traits such as modesty, charity, kindness and the like. The way to determine the value of a character trait is to determine its impact on our moral sentiments (which desirism holds to be false). Furthermore, the favorable impact on our moral sentiments is determined by whether the trait in question is agreeable to the self, useful to the self, agreeable to others, or useful to others.
If these "character traits" are taken to be desires, and their agreeableness or usefulness to self or others is taken to be the degree to which a desire tends to objectively satisfy directly or indirectly one’s own desires and the desires of others, we get from Hume a statement that is very close to desirism.
Desirism would agree that whether we approve or disapprove of something depends on its (subjective) impact on our own desires. However, desirsm answers this issue with G.E. Moore’s distinction between what people do approve of and what people should (or have reason to) approve of.
What people do approve of is what impacts their sentiments given their beliefs. What they have reason to approve of, and what they have reason to cause others to approve of, and what others have reason to cause them to approve of, is found in the criteria of usefulness and agreeableness to self and others.
Reason Is the Slave of the Passions
Hume wrote in Treatise on Human Nature
Reason is, and ought only to be the slave of the passions, and can never pretend to any other office than to serve and obey them.
There is a lot of controversy about what exactly Hume was trying to say here. However, in this sea of interpretations, there are some that come close to what desirism claims.
Desirism holds that there is no truth content to desires and no way to evaluate a desire as “correct” or “incorrect” on its own. We can only evaluate a desire in terms of its effects in aiding or interfering with the objective satisfaction of other desires. Desires, in this sense, are the root of all values – even when it comes to evaluating (other) desires.
Beliefs have a truth value. A belief is correct when there is a fact of the world that corresponds to a belief. An agent’s belief that an invisible ancient dragon watches his every move is an accurate model of the world in which he lives if it happens to be the case that there is an invisible ancient dragon that watches his every move.
A desire may be objectively satisfied if the proposition identifying what is desired is true. However, there is nothing in this that identifies a desire as being "correct" or "incorrect".
When it comes to deciding on a course of action, desires determine the end or goal of the action. Desires dictate where the action is headed – what the final objective is. Once desires have set the goal, beliefs go to work selecting the means to reaching that goal. In this sense, beliefs work to serve desires.
Sentiments (Desires) Cannot Be Intrinsically Unreasonable
Reason is something that belongs to the realm of beliefs. Its purpose is to help belief to carry out the commands of desires.
Consequently from Treatise on Human Nature:
Tis not unreasonable for me to prefer the destruction of the whole world to the scratching of my finger.
After saying this, it is relevant to note that there is a potential for conflict among desires. In these cases, reason may argue that, to fulfill the will of some desires, other desires should either be strengthened or diminished. Reason can dictate that a person is better off with no desire to smoke – given that such a desire must either be frustrated, or satisfied to the frustration of other desires brought about by poor health, a long and debilitating illness, or an early death.
Thus, perhaps we should modify the quote above to say that it is not necessarily unreasonable to prefer the destruction of the whole world to the scratching of one’s finger. However, given that the destruction of the world, even if desired, would thwart other desires, an agent may be better off discouraging in himself as well as in others a fondness for the destruction of the world.
However, even in this type of case, the motivation to rid oneself from a desire – the sentiment that marks some desires as bad or best gotten rid of - does not come from reason alone. Here, too, reason is continuing to play its role as servant to the other desires – the master desires – with which the judged desire comes into conflict.
One way often used to understanding this difference between beliefs and desires is as follows: If a proposition that is believed is not true in the world, we should change the belief. However, if a proposition that is desired is not true of the world, we have a reason to change the world.
Relationships Between Sentiments and Objects of Evaluation Are Matters of Fact
As mentioned above, desirism denies that morality depends on the sentiments of the evaluator.
Furthermore, desirism allows that moral claims have a truth value that reason can help us to determine. There is a fact of the matter as to whether a desire tends to be agreeable and useful to self and others that reason can help us to determine.
It is still the case that these moral facts cannot exist in a world without passions. Relationships between states of affairs and desires (passions) cannot be known by reason or by any other means in a world where no passions exist. However, in a world where there are passions, the relationship between objects of evaluation and those passions are knowable.
Many would say that this conflicts with Hume. However, this may not be the case.
There is a point at which Hume discusses relationships between objects of evaluation and desires where he states:
Take any action allowed to be vicious: Willful murder, for instance. Examine it in all lights, and see if you can find that matter of fact, or real existence, which you call vice. In whichever way you take it, you find only certain passions, motives, volitions and thoughts. There is no other matter of fact in the case. The vice entirely escapes you, as long as you consider the object. You never can find it, till you turn your reflection into your own breast, and find a sentiment of disapprobation, which arises in you, towards this action.
Yet, if you were to continue this to the very next sentence – the words people cut off in trying to argue that Hume denied the possibility of moral facts – one gets:
Here is a matter of fact; but it is the object of feeling, not of reason.
That is to say, relationships between objects of evaluation and the sentiments are matters of fact. They can be expressed as true propositions. The truth of the proposition can be known in the same way that any other proposition can be known. However, they are facts about feelings and their objects. They are not facts about objects independent of feelings and discoverable by reason alone.
Desirism is quite comfortable with these claims.