How do we distinguish intrinsic (final) value from merely instrumental value? Simon Keller argues that relationships are of merely instrumental value on the grounds that (i) unhappy relationships which lack all instrumental value seem to have no value at all; (ii) we don't seem to have any reason to create new relationships (e.g. between happy hermits) that would have no further beneficial consequences; and (iii) the badness of bad relationships is typically explained reductively ("it's making you both miserable"). Even granting these claims for sake of argument, I'm not convinced it follows that relationships are of merely instrumental value. That would imply that they are fungible; but a value could be contingent in all the ways specified above without thereby being fungible.
For clarity, let's translate value-talk into reasons-talk (cf. "buck passing"), such that 'X is valuable' becomes 'X is desirable', i.e. there are reasons to want X. If X has merely instrumental value, then the reasons to want X are wholly parasitic on our reasons to want some other Z (to which X is conducive). That is, we have no reason to want X over and above our reasons to want Z. Merely instrumental value thus consists in there being reasons only for instrumental desire. Reasons to take something as an ultimate end, by contrast, amount to "intrinsic" or final value.
Now, why think that instrumental value must be fungible? Intuitively: one means is as good as any other, so long as it comes to the same in terms of our final goals. If we have no reason to want X over and above our reasons to want Z, then the fact that the latter goal is served just as well by some alternative means Y (without detrimentally impacting our other goals) entails that we can have no reason to prefer X to Y. It's just like swapping two $10 bills for a $20.
But it seems bizarre to think that relationships are fungible in this way. Most people would not wish to swap spouses with a stranger, even if they knew this would have no effect on how happy (etc.) each person was. Even more dramatically, suppose that one's spouse was long ago kidnapped and secretly replaced by a cleverly disguised robot. (The real person is then set up in a duplicate world with a cleverly disguised robotic copy of you, etc.) This is clearly a terrible outcome. But all the instrumental benefits remain as before: each robospouse provides their human partner with happiness, and promotes their moral flourishing, etc. What's missing is the genuine personal connection. Since the loss of it makes no instrumental difference, the value here must be intrinsic.
What, then, should we make of Keller's three arguments? I think they rest on a failure to distinguish the content of a reason and its preconditions. This is clearest in the case of (i) and (iii). Contingency of desire does not imply instrumentality -- as explained in my old Railton-inspired post on 'Regulating Aims' -- and likewise for desirability. Some X may be non-instrumentally desirable only if it leads to Z. Given the absence of fungibility here, this is not the same as X being desirable merely as a means to Z.
(We may also distinguish having Z-contingent or 'conditional' reasons to desire X straight out, versus having straight-out reason to conditionally desire X given Z. But since neither of these is merely instrumental, the distinction is not essential for present purposes.)
So the mere fact that instrumentally worthless ("unhappy" for short) relationships have no value at all does not imply that the value of other relationships is merely instrumental. It may be that only if a relationship is conducive to happiness do we have reason to want to maintain it; but nonetheless, what this reason mandates is a non-instrumental desire to maintain this very relationship, and not just any old means to equal happiness.
Of course, if the reason were contingent in this way, that would also suffice to make sense of why one could reductively explain the undesirability of the relationship by citing the absence of this precondition. "This relationship is bad because it is making you both miserable" implies that if it were happier then you'd have reason to desire the relationship. It does not imply instrumentality (or that you would have reason to desire the relationship only for the happiness etc. it would bring): again, a contingent reason might just as well mandate a non-instrumental desire.
So (i) and (iii) are invalid grounds for inferring instrumentality. This post is too long already, so I'll leave discussion of (ii) for the next post...