Alice and Bob are both perfect Bayesian epistemic agents and
subjectively perfect utilitarians (i.e., they always do what by their
lights maximizes expected utility). Bob is going to Megara. He comes to
a crossroads, from which two different paths lead to Megara. On exactly
one of these paths there is a man-eating lion and on the other there is
nothing special. Alice knows which path has the lion. The above is all
shared knowledge for Alice and Bob.
Suppose the lion is on the left path. What should Alice do? Well, if
she can, she should bring it about that Bob takes the right path,
because doing so would clearly maximize utility. How can she do that? An
obvious suggestion: Engage in a conventional behavior indicating a where
the lion is, such as pointing left and roaring, or saying “Hail well-met
traveler, lest you be eaten, I advise you to avoid the leftward leonine
path.”
But I’ve been trying really hard to figure out how is it that such a
conventional behavior would indicate to Bob that the lion is on the left
path.
If Alice were a typical human being, she would have a habit of using
established social conventions to tell the truth about things, except
perhaps in exceptional cases (such as the murderer at the door), and so
her use of the conventional lion-indicating behavior would correlate
with the presence of lions, and would provide Bob with evidence of the
presence of lions. But Alice is not a typical human being. She is a
subjectively perfect utilitarian. Social convention has no normative
force for Alice (or Bob, for that matter). Only utility does.
Similarly, if Bob were a typical human being, he would have a habit
of forming his beliefs on the basis of testimony interpreted via
established social conventions absent reason to think one is being
misinformed, and so Alice’s engaging in conventional left-path
lion-indicating behavior would lead Bob to think there is a lion on the
left, and hence to go on the right. And while it woudl still be true
that social convention has no normative force for Alice, Alice would
think have reason to think that Bob follows convention, and for the sake
of maximizing utility would suit her behavior to his. But Bob is a
perfect Bayesian. He doesn’t form beliefs out of habit. He updates on
evidence. And given that Alice is not a typical human being, but a
subjectively perfect utilitarian, it is unclear to me why her engaging
in the conventional left-path lion-indicating behavior is more evidence
for the lion being on the left than for the lion being on the right. For
Bob knows that convention carries no normative force for Alice.
Here is a brief way to put it. For Alice and Bob, convention carries
no weight except as a predictor of the behavior of convention-bound
people, i.e., people who are not subjectively perfect utilitarians. It
is shared knowledge between Alice and Bob that neither is
convention-bound. So convention is irrelevant to the problem at hand,
the problem of getting Bob to avoid the lion. But there is no solution
to the problem absent convention or some other tool unavailable to the
utilitarian (a natural law theorist might claim that mimicry and
pointing are natural indicators).
If the above argument is correct—and I am far from confident of that,
since it makes my head spin—then we have an argument that in order for
communication to be possible, at least one of the agents must be
convention-bound. One way to be convention-bound is to think, in a way
utilitarians don’t, that convention provides non-consequentialist
reasons. Another way is to be an akratic utilitarian, addicted to
following convention. Now, the possibility of communication is essential
for the utility of the kinds of social animals that we are. Thus we have
an argument that at least some subjective utilitarians will have to
become convention-bound, either by getting themselves to believe that
convention has normative force or by being akratic.
This is not a refutation of utilitarianism. Utilitarians, following
Parfit, are willing to admit that there could be utility maximization
reasons to cease to be utilitarian. But it is, nonetheless, really
interesting if something as fundamental as communication provides such a
reason.
I put this as an issue about communication. But maybe it’s really an
issue about communication but coordination. Maybe the literature on
repeated games might help in some way.