The unwilling addict has an overwhelming first-order desire for heroin, but wishes that this were otherwise. How are we to explain this second-order desire? Presumably it is not an intrinsic, brute desire. We might think it instrumental: the addict desires good health, etc., and ridding himself of his desire for heroin would help bring about this desired end. But here's a puzzle: in that case his first-order desire for heroin, being stronger than his desire for health, should give rise to an equally overwhelming second-order instrumental desire to be rid of his desire for health (for the sake of the more-desired end of using heroin). As pointed out in class last week, the first-order weightings should replicate themselves all the way up the hierarchy, if higher-order desires are instrumental. After all, instrumental desires are not really additional desires at all; they are simply the pairing of an underlying (intrinsic) desire with a means-end belief. They are in this sense 'transparent' -- mere placeholders for an underlying desire, which provides the entire motivational force (filtered by one's degree of credence in the means-end belief). So understood, instrumentalism cannot explain the emergence of substantive desire hierarchies, where our higher-order desires govern (rather than merely reflect) our first-order ones.
Yet even if higher-order desires cannot be purely instrumental, it seems clear that they depend in some sense on first-order desires. Plausibly, his desire for health is the reason why the unwilling addict is unwilling. There is some explanatory relation here. But what? We might opt for a mere causal-historical explanation: the desire for health happened to give rise (through some non-rational process or psychological quirk?) to a new (intrinsic) 2nd-order desire to be rid of the addiction. But this implies that the desires are now independent, so even if he ceased to care about his health and all that, the addict would retain his brute desire to be rid of the addiction. That doesn't sound right.
Is there a third option? Perhaps the desire for health not only created, but also sustains, the second-order desire. Can we make sense of this proposal without collapsing it into the first failed account (i.e. transparent instrumentalism)? I think we can, on the model of conditional desires. The higher-order desire to be rid of addiction is implicitly conditional on one's desiring of other goods in life (e.g. health). If one loses those other desires, then the higher-order desire is neither thwarted nor satisfied, but simply 'cancelled' -- it no longer applies. Nonetheless, so long as the condition (of one having those other desires) is met, this does count as a further desire in addition, with weight of its own that is not simply inherited from others.
I should note that this still doesn't answer the comparative question why one came to have this higher-order desire rather than some other; but perhaps that is not a fact in need of explanation.* What this account succeeds in explaining is how higher-order desires can in some sense depend upon first-order desires, without merely replicating their weightings (as instrumental desires would, given their 'transparent' nature).
* = Having said all that, I think we would do better yet to abandon strict Humeanism and allow that value judgments can give rise to desires. The stronger higher-order desire to be rid of the addiction thus reflects our cognitive judgment that health (etc.) is better than heroin.
Actually, I'm inclined to deny that the addict really desires heroin (non-instrumentally). It seems more plausible to say he desires to be rid of the unpleasant craving sensation, and either taking heroin or somehow losing the addiction would equally satisfy this negative desire. The difference between 'willing' and 'unwilling' addicts is then not a matter of second-order desire at all, but simply a refinement of the negative first-order desire, by further specifying which of the two possible means is preferred. (Maybe I'm working with an overly intellectualized sense of 'desire' here, but I guess that depends on what you want to do with the concept.)