Many people reading my postings on desirism do so under the assumption that it must start with a set of fundamental moral commandments. With this in mind, they then search for an interpretation that is consistent with this assumption. They find their commandment, then set about to criticize it.
However, their fundamental assumption is wrong, which means that their interpretation is incorrect. Consequently, the theory the criticize is not the theory that I wrote.
"Desires are the only reasons that exist" is not a commandment. It is a fact about the world. I do not see any evidence to support claims for the existence of any type of reason for action other than desires.
Also, desirism holds that "should" questions can only be answered by appeals to reasons for action. "Why should I do X?" No answer makes sense that is not a reason for action, or a fact that ties some consequence of the action to a reason for action.
If we tie these two claims together, we get the conclusion that all answers to "should" questions must, directly or indirectly, reference one or more desires. Where those claims appeal to facts, those facts are made relevant (or irrelevant) to the degree that they relate (or fail to relate) some state of affairs to desires.
This still is not a commandment. This is still a statement about what is true or false – though it is a statement about what is true or false regarding “should” statements.
Somebody who wants to dispute these claims either needs to explain how something other than a reason for action or facts that tie consequences to reasons for action can answer a "should" question. Or they need to show evidence of reasons for action that exist other than desires.
A third possible response, of course, is to show that there is a third option that I am overlooking. Between these three, this should exhaust the total set of possible answers.
On the other hand, if this position stands, the next question to ask is: “What follows from these facts that “should” questions are answered by appeals to reasons for action that exist and desires are the only reasons for action that exist?”
But this is a different set of questions. I could be wrong about the implications of these two claims – without being wrong about the claims themselves. So, objections raised to the implications that I draw from these claims do not imply that the claims themselves are false.