Consider a Humean package view of rationality where:
Then end of practical rationality is desire satisfaction.
All the rational motivational drive in our decisions comes from our desires.
There are no rational imperatives to have desires.
Now suppose that you learn that some costless action will further one or more of your desires, but you have no idea which desire or desires will be furthered by that action. (If we want to have some ideality constraints on which desires make action rational—say, only desires that would survive idealized psychotherapy—then we can suppose that you also know that the desire or desires furthered by the action will satisfy those constraints. I will ignore this wrinkle.)
Any theory of rationality that holds it to be rational to pursue one’s desires should hold it both rational and possible to take that costless action. In the abstract, a case where you know that some desire will be furthered but have no idea which one seems a strange edge case. But actually there is nothing all that strange about this. When money is offered to us, sometimes we have a clear picture of what the money would allow us to do. But sometimes we don’t: we just know that the money will help further some end or other. (Of course, in some people, the pursuit of money may have a non-instrumental dimension, but that’s vicious and surely unnecessary.)
So now let’s go back to the costless action that furthers one or more of your desires and the desire theory of rational motivation. How can this theory accommodate this action?
Option 1: Particular desires. You pick some desire of yours—let’s say, a desire to read a good book—and you think to yourself: “There is a non-zero probability that the action furthers my desire to read a good book.” Then the desire to read a good book, in the usual end-to-means ways, motivates you to do the costless action.
That, of course, could work. And in fact, in the case of money we do sometimes proceed by imagining something that we could buy. However, thinking that what motivates one is just the non-zero probability of furthering a particular desire gets things wrong for two reasons. The first is that we could imagine the case being enriched by your learning that the desire that will be furthered by the action is none of the desires that would come to mind if you were to spend less than a minute thinking about the case but that you need to make your decision within a minute. The inability to think of a particular desire that even might be furthered by the action does not affect the ratioanl possibility of taking the costless action.
The second is that this approach gets the strength of motivation wrong. You have many desires, and the desire to read a good book is only one among many. The probability that that desire to read a good book would be furthered by the costless action might well be tiny, especially if you received the further information that it is only one of your desires that is furthered by the action. Such a small probability of a benefit could still motivate you to take a costless action, but it may not work for similar cases where there is a modest cost. For instance, we can suppose you learn that:
The benefit is roughly equal to reading a good book as measured by desire-satisfaction.
The cost is roughly equal to a tenth of the benefit of reading a good book as measured by desire-satisfaction.
You have a hundred desires and the one furthered is but one of them.
Well, then, the action is clearly worth it by (4) and (5). But it’s not worth doing the action simply on a one percent chance that it will lead to reading a good book, since the cost is ten percent of the benefit of reading a good book.
One might try to remedy the second problem by mentally going through a larger number of desires so as to increase the probability that some one of the desires will be fulfilled. But we still have the first objection—there may not be enough time to do this—and surely it is implausible that one would have to go through such mental lists of desires in order to get the motivation.
Option 2: A higher-order desire to have satisfied desires. Suppose you have a higher-order desire H to satisfy lower-order desires. Then while you don’t know which lower-order desire is furthered by the action, you do know that this higher-order desire is furthered by them.
This approach seems to lead to an unfortunate double-counting. When you sit down to read a good book, do you really get two benefits, one of reading the book and the other of furthering the higher-order desire to have satisfied lower-order desires? If not, the approach is problematic. But if so, then it gets the rational strength of motivation wrong. For suppose that you are choosing between two actions. Action A will lead to your reading a good book. Action B will lead to the fulfillment of an unknown desire other than reading a good book, a desire you nonetheless know to have the same weight. On the higher-order solution, it seems you have a double motivation for action A, namely H and the desire to read a good book, but only a single motivation for action B, namely H, and hence you should have a twice as strong rational motivation for A. But that’s surely not rational!
Maybe, though, you can get out of the double-counting in some way, by having some story about desire-overlap, so that H and the desire to read a good book don’t add up to a double desire. I suspect that this may undercut the force of the story, by making H not be a real desire.
But there is a second and more serious problem with the story. Suppose that Jim has all the usual lower-order order desires but lacks H. If rational motivation comes from desires, then Jim will not be rationally motivated to the action. (Maybe he will have some accidental non-rational motivation for the action.) But surely not going for a costless action that he knows will fulfill some desire of his will be a rational failing, assuming that it’s rational to fulfill one’s desires. Hence there will have to be a rational imperative to have H among one’s desires, contrary to the third part of the Humean package we are exploring.
Now I suppose we could drop the third part of the Humean picture, and hold that rationality requires some desires like H. But I think this makes the rest of the picture less plausible. If rationality requires one to have certain desires, it could just as well require one directly to fulfill certain ends, thereby undercutting the second part of the Humean picture.
Finally, I should note that not all non-Humeans should rejoice at this argument. For similar considerations may apply against some other views. For instance, some Natural Law views that tie motivation very tightly to basic goods may have this problem.