There is a battle being fought between competing camps concerning what a desire is.
I have recently discussed the position that the Humean dispositionalist theories are defending. This is the position that a desire that p is a disposition to act so as to make it the case that p.
I have agreed that there is a relationship between a desire that p and a disposition to act. However, the relationship is that of cause and effect. The desire causes the disposition, it is not identical to the disposition. In the same way, rain causes plants to grow, it is not identical to the growing of plants.
I have three major objections to dispositional theories.
First, there are clearly dispositions to act that are not desires. There are habits. You can perform this experiment at home. Reprogram your computer keyboard to switch the 't' and 'r' keys. If you are at all skilled at typing, you will find yourself typing 'tre' when you intend to type 'the'. This is an intentional action - an action as intentional as typing 'the' was before reprogramming the computer. Furthermore, you have the requisite beliefs. I am assuming that you programmed the keyboard yourself and know that you switched it. On the dispositional theory of desire, you must desire to type 'tre'. However, that is false. So, there are dispositions to act that are not desires. Reducing desires to dispositions to act fails.
Second, the disposition to act principle implies that having a desire that p is incompatible with having the belief that p. It is absurd to attribute to you a disposition to bring about p if you believe that p is already true. Consequently, the desire to bring about p must disappear the moment you come to realize that p is already the case. This is called the "death of desire principle". However, desires do not work this way. As I prepare supper, it is not the case that I have a desire to eat that suddenly vanishes the moment I start eating. Nor does my desire to cuddle with my wife vanish the moment that I start cuddling with my wife. Dispositionalist respond to this type of situation by saying that the agent has a new desire - a desire to continue eating or a desire to continue cuddling. Yet, even this is problematic. It requires that one desire vanish and a new desire suddenly emerge, rather than the more plausible assumption that the same desire survives the transition.
Third, dispositional theories become exceptionally complex very quickly as we add more desires. Assume that you had a balance with a number of weights. Let us assume that there are 26 weights lettered A through Z, each having a different weight. If we were to describe the effects of these weights dispositionally, the descriptions will be very complex. Weight T, when combined with weights H and J, will tilt the balance in its favor when countered by weights X and Z, but not when countered by weights X and W. We would need to express this type of conditional for all different combinations of weights, and that is assuming an accurate scale. It is far easier to attribute a value to each weight and simply say, "the balance will tilt in the direction of the side with the highest total weight." However, "assigning a value" to each weight is an evaluativist move. Evaluative theories are the leading opponent to dispositional theories. "Assigning a value" means abandoning dispositional theories in favor of some sort of evaluativist theory.
The assignment theory of desire can handle all three of these problems.
It does not call the typing of 'tre' a desire because the agent does not assign any importance to it being the case that he has typed 'tre', or any consequence of typing 'tre'.
It attributes a value to a state such as "I am eating" or "I am cuddling with my wife" that makes it important to the agent to make or keep the proposition true. This means making the proposition true while it is false, and keeping it true while it is true. These desires do not disappear once the agent realizes that he has realized his desired end.
By assigning a value - a weight, or a measure of importance - to different desires the evaluativist theory makes it easier to use desires to explain and predict intentional behavior. Or, more to the point, it explains the fact that we can so easily use desires to explain and predict intentional behavior.
For these three reasons, an assignment theory of desire is superior to a dispositional theory of desire.
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