Today I am going to argue against something that the bulk of philosophers who write on reasons for action seem to accept.
The claim is that "Desires are a necessary requirement for reasons; but they are not sufficient."
I will argue that having a desire is a sufficient condition for having a reason. If you are trying to answer the question, "What Should I Do?" every desire provides a reason proportional in strength to the strength of desire.
It does not necessarily provide a good reason. In order to determine the quality of the reason we are going to have to look at how well it fits in with other reasons for action - whether it will tend to fulfill other desires or thwart them. However, desires provide reasons - good reasons, bad reasons, and reasons of neutral value.
The aversion to pain is a sufficient reason to avoid pain - even in those circumstances where pain does not do any good. You know that physical therapy is going to be painful. The pain is doing nothing but giving you are reason to avoid physical therapy. However, it IS giving you a reason to avoid physical therapy. It may be outweighed by more and stronger reasons to have physical therapy but, in the absence of those other reasons, one would have a perfectly legitimate reason not to go to phyisical therapy.
Why do philosophers agree that having a desire is necessary, but not sufficient, for having a reason?
The arguments come from a huge set of apparent examples where we can imagine that a person has a desire but we judge that the agent does not have a reason to perform the action.
From the Standford Encyclopedia of Philosophy article on Reasons: Internal and External http://plato.stanford.edu/entries/reasons-internal-external/#DirExtArg
The literature is also full of extensional arguments against theories which resemble internalism, but on the grounds that they overgenerate, rather than undergenerate, reasons. Many famous and colorful examples—about people who want to eat saucers of mud, or count blades of grass, or who have a disposition to turn on radios—are offered to show that not every desire or motivation is of the right kind to generate practical reasons. Strictly speaking, however, such cases only create objections to views which postulate a sufficient condition for the existence of reasons, and internalism itself postulates only necessary conditions, and no such sufficient condition, as Bernard Williams makes clear (1989), for example.
Why do I object to this argument?
It is because I do not see a difference between these desires and other desires that people take to provide perfectly legitimate reasons for action.
Let us take, for example, the desire to "eat saucers of mud".
Earthworms eat mud . . . or, dirt, to be more precise, but mud is a form of dirt that earthworms will eat.
Noting this fact, I have no difficulty imagining an extraterrestrial race that has acquired a desire to eat mud. As we sit around the bargaining table in orbit above Saturn with them working out an interstellar trade ready, we sit with our bagels and muffins while they sit with their saucers of mud and we work out the details of the treaty.
One could argue that this is different. They have a reason to eat mud - because of its food value.
However, this alien race has something in common with humans. They tend to over-eat. They will eat their saucers of mud even when they are not eating it for its food value, just as we continue to consume more bagels and muffins than we need to maintain good health.
When it gets to the point that they are eating saucers of mud for no reason other than because they like to, they are no different than us when we get to the point that we are eating bagels and muffins for no reason other than because we like to.
Nature has given them a desire to eat mud as an end in itself - as a goal - not as means for maintaining health, and so they continue to eat mud even beyond the point that it provides good health. It is no different than the desire we have to eat bagels and muffins - not as a means to good health, but as an end in itself - which motivates us to eat bagels and muffins even when it does not support good health.
Sex works the same way. We evolved to have a desire for sex because it contributes to the procreation of the species. However, we evolved to have a desire for sex as an end in itself - not as a means for procreation. Thus, we engage in sex even in circumstances where we do not seek reproduction. In fact, it is almost certainly the case that the bulk of our sex acts take place in conditions where we know procreation to be impossible or are actively seeking to avoid procreation.
In order to be consistent and coherent, philosophers either have to admit that desires to eat saucers of mud, count blades of grass, and the like are sufficient to generate reasons for intentional action for those people who have them, or assert that our desires for food or sex beyond that which to maintain physical health and propagate the species fail just as badly to provide reasons for action.
By the way, there is nothing special about "maintaining good health" or "propagating the species". The role that having these desires has in propagating the species explains how we evolved to have them, but it does not give them value. Even maintaining good health and propagating the species has value (is something we have reason to do) in virtue of the fact that they fulfill desires and for no other reason. The reason why I looked at examples of eating saucers of mud or bagels beyond what was necessary for health was simply to avoid these complications.
Ultimately, there is nothing else in the universe - nothing that exists - that provides end-reasons for intentional action except desires so, if desires cannot provide reasons, reasons do not exist.
Why do philosophers think that these provide such good examples against the idea that desires are sufficient to provide reasons?
I think that it comes from a sort of prejudice. A philosopher - a human being - looks at the imaginary person with a desire to eat dirt as something that she does not wish to be. Her aversion to being that type of person (itself a desire) causes her to dismiss the interests (desires) of that imaginary person as illegitimate - as something incapable of providing actual reasons.
In a slightly different case, an individual can look upon a homosexual as something that he does not want to be. Because of his aversion to being a homosexual, he dismisses the homosexual interest as sick, perverse, as "failing to provide a reason to engage in homosexual acts." Consequently, the interests of the homosexual as a homosexual can be dismissed - it cannot provide a reason to adopt policies or practices that would respect those interests.
But, in fact, there is nothing behind this judgment but the agent's own aversion to being that kind of person. There is no argument for this position - just the expressions of approval given to those who agree and expressions of disapproval given to those who disagree.
We tend to dismiss interests that are not our own. When we can get together with other people who also do not share a particular interest, we can usually agree with each other to dismiss it as irrelevant as providing "no reason" to act differently.
It is relevant at this point to remind the reader that this digression is taking place in a discussion of J.L. Mackie's Ethics: Inventing Right and Wrong. Mackie argued that the distinction between the current (error-filled) moralizing and a new and improved error-free moralizing rests precisely in tossing out our tendency to dismiss interests that are not our own - precisely because the difference we imagine finding between these different types of interests do not exist in the real world.
I will repeat - desires do not necessarily provide good reasons for action. Whether those reasons are good or bad depends on their tendency to fulfill or thwart other desires. However, even bad reasons are still reasons (that are bad), in the same way that bad apples are still apples (that are bad). It would be great if bad reasons did not exist (just as it would be great if bad apples do not exist). But they do, and they are found in desires that tend to thwart other desire.
Still, I note that this is a minority position. It is not a position that is shared by many people who write on this subject.