Doug S. is proposing a possible reductio ad absurdum of desirism.
He wishes to argue that the propositions of desirism conclude that all actions are good because they can all be described as actions that a person with good desires would do,
[A]ssuming that X is a sufficiently good desire, then "X, except in some over-specified, rare situation that most people won't ever be in, in which case Y" should also be a good desire, because it's still good "in general". So, if "don't murder anyone" is a good desire, then "don't murder anyone, except under this extremely constrained situation that almost nobody will ever encounter" should also be a good desire.
Every possible act in the real world happens at a specific place and time. So, by simply specifying that place and time precisely enough, I can take any desire, add a sufficiently narrow exception, and still have a good desire. Therefore I can take any agent with good desires, change those desires by adding a narrow exception, and end up with an agent with good desires that will performs the act I specified. Since I can do this for any act at all, that means that every possible act has at least one possible agent with good desires which will perform that act. So all acts are right acts.
An example was brought up in discussion of the killing of a little girl at a specific place in time. If an aversion to killing is a good desire, then an aversion to killing except when killing this young girl at this specific place and time would be an almost equally good desire. So, the killing of this girl may well be something that a person with good (though not perfect) desires would perform.
In response to this I wish to bring forth what desirism says is the relationship between reason and morality. The role of reason to morality is the same as the role of reason to fixing the flat tire on your car. You can reason with the flat tire all you want, but you will not convince it to change places with the spare. The role of reason is to tell you how to use the tools at your disposal to change the flat tire.
Just as you cannot reason the flat tire into changing itself, you cannot reason a person into virtue. Reason is the instrument for altering a person's beliefs. Virtue is a quality of malleable desires. To alter people's desires you do not use reason, you use the tools of praise, condemnation, reward, and punishment. However, reason will tell you how best to use these tools.
Praise and condemnation are core moral concepts. In desirism, they explain why moral concepts are not applied to fixed desires but to malleable desires - because fixed desires are by definition immune to the effects of cultural tools. It also explains why the concept of an evolved morality is nonsense - because it makes no sense to morally praise or condemn a person for the sequence of their genes. Praise, condemnation, and other cultural tools can have no effect on those sequences. No person can justly claim moral superiority over another in virtue of having a morally superior genetic sequence.
When we bring these concepts to bear on Doug's desires, one of the questions we need to ask is what the likely effects would be of cheerfully praising the killing of this girl. Will this cheerful praise actually help to promote an aversion to killing except for the killing of this girl at this particular place and time? Or will it likely have the effect of weakening the desire for killing generally?
I would suggest that it is the latter. We have more to gain in society as a whole by condemning the killing of this young girl and thereby promoting an overall aversion to the killing of young children, then we have by praising the killing of this young girl and promoting a weakened aversion to killing young girls that would put others at risk of the same fate.
We can clearly invent stories in which actions which we have reason to condemn are not actions that the people in those situations have reason to condemn. However, this is not a threat to desirism. This is, in fact, a part of desirism.
Imagine a planet in which a species much like humans evolved. However, on that planet, there came to exist a sexually transmitted virus. This virus, as it does out, does not do harm to those who are infected. It provides a benefit. Let us say that it forms a symbiotic relationship with the infected person's immune system that makes it more effective against some particularly harmful diseases.
On this planet, pre-historic tribal cultures that tolerated and encouraged sex between adults and children had the effect of infecting children with this virus. These cultures grew and prospered. Where those cultures that condemned this practice ended up with sizable numbers of children getting sick and dying. The cultures that exist today on this planet are the descendants of those historic cultures that condoned the practice of sex with children and even required the rape of children who would otherwise refuse sex.
Consider the similarities between the rape of a child in this hypothetical world and providing a child with a vaccination. Both involve the violent penetration of a child's body against the child's will. It takes a minor change to make an act of sex the delivery system for such rather than an act of skewering the child. The latter, of course, is an act that is not only permissible in our society, but (arguably) obligatory.
However, none of this implies that we humans living on Earth today should praise the act of having sex with children. What is true on that far distant planet is not true on Earth. The fact of the matter is that we, living in the real world, have many and strong reasons to condemn those acts in order to weaken the desire to perform such acts and to put up conflicting desires that would inhibit and reduce the rates at which people engage in such acts.
We may be uncomfortable with the thought that such acts might be permissible where different facts obtain. We are supposed to be uncomfortable with the thought. Part of what it means to promote an aversion to such states is that one is promoting a feeling of discomfort at the thought of such states obtaining and a desire to condemn or punish those who realize such states.
I can even go so far as to say that we have reason to worry about – and even to morally condemn – anybody who does not have an averse emotional reaction to the thought of such a world exist. They are likely to be a threat to us and those we care about.
However, the fact of - and even the justification for - that discomfort for us in this situation does not imply that, if the facts were different, there justification for that discomfort might disappear with it. It justifies our not liking that fact. It may even demand that we have an adverse emotional reaction to the possibility of such facts. However, it does not alter the facts.
In the real world, in the here and now, if somebody were to try to argue for praising the killing of this young girl on the grounds that, "it promotes an aversion to killing except in this one specific instance and that, generally, is a good aversion to promote," we would justly claim that the person is mistaken. The real world isn't built that way. In the real world, his praise of that young girl's death weakens the aversion to killing generally and puts others at risk. The praise of such a killing, like the killing itself, is something we in the real world have many and strong reasons to condemn.