Suppose I observe some piece of evidence, attempt a Bayesian update of my credences, but make a mistake in my calculations and update incorrectly. Suppose that by luck, the resulting credences are consistent and satisfy the constraint that the only violations of regularity are entailed or contradicted by my evidence. Then I realize my mistake. What should I do?
The obvious answer is: go back and correct my mistake.
But notice that going back and correcting my mistake is itself a transition between probabilities that does not follow the Bayesian update rule, and hence a violation of the standard Bayesian update rule.
To think a bit more about this, let’s consider how this plays out on subjective and objective Bayesianisms. On subjective Bayesianism, consistency, the Bayesian update rule and perhaps the constraint that the only violations of regularity are entailed or contradicted by my evidence. My new “mistaken” credences would have been right had I started with other consistent and regular priors. So there is nothing about my new credences that makes them in themselves rationally worse than the ones that would have resulted had I done the calculation right. The only thing that went wrong was the non-Bayesian transition. And if I now correct the mistake, I will be committing the rational sin of non-Bayesian transition once again. I have no justification for that.
Moreover, the standard arguments for Bayesian update apply just as much now in my new “mistaken” state: if I go back and correct my mistake, I will be subject to a diachronic Dutch Book, etc.
So, I should just stick to my guns, wherever they now point.
This seems wrongheaded. It sure seems like I should go back and fix my mistake. This, I think, shows that there is something wrong with subjective Bayesianism.
What about objective Bayesianism? Objective Bayesianism adds to the consistency, update and (perhaps) regularity restrictions in subjective Bayesianism some constraints on the original priors. These constraints may be so strict that only one set of original priors counts as permissible or they may permissive enough to allow a range of original priors. Now note that the standard arguments for Bayesian update still apply. It looks, thus, like correcting my mistake will be adding a new rational sin to the books. And so it seems that the objective Bayesian also has to say that the mistake should not be fixed.
But this was too quick. For it might be that my new “mistaken” posteriors are such that given my evidential history they could not have arisen from any permissible set of original priors. If so, then it’s like my being in possession of stolen property—I have posteriors that I simply should not have—and a reasonable case can be made that I should go back and fix them. This fix will violate Bayesian update. And so we need to add an exception to the Bayesian update rules: it is permissible to engage in a non-Bayesian update in order to get to a permissible credential state, i.e., a credential state that could have arisen from a permissible set of priors given one’s evidential history. This exception seems clearly right. For imagine that you are the mythical Bayesian agent prior to having received any evidence—all you have are your original priors, and no evidence has yet shown up. Suddenly you realize that your credences violate the objective rules on what the priors should be. Clearly you should fix that.
Thus, the objective Bayesian does have some room for justifying a “fix mistakes” exception to the Bayesian update rule. That exception will still violate the standard arguments for Bayesian update, and so we will have to say something about what’s wrong with those arguments—perhaps the considerations they give, while having some force, do not override the need for one’s credences to be such that they could be backtracked to permissible original priors.
Considerations of mistakes gives us reasons to prefer objective Bayesianism to subjective Bayesianism. But the objective Bayesian is not quite home free. Consider first the strict variety where there is only one permissible set of original priors. We have good empirical reason to think that there are about as many sets of original priors as there are people on earth. And on the strict version of objective Bayesianism, at most one of these sets of original priors is permissible. Thus it’s overwhelmingly unlikely that my original priors are permissible. Simply fixing my last mistake is very unlikely to move me to a set of posteriors that are correct given the unique set of permissible original priors and my evidential history. So it’s a matter of compounding one rational sin—my mistake—with another, without fixing the underlying problem. Maybe I can have some hope that fixing the mistake gets me closer to having posteriors that backtrack to the unique permissible original priors. But this is not all that clear.
What about permissible objective Bayesianism? Well, now things depend on our confidence that our original priors were in fact permissible and that no priors that generate our new “mistaken” posteriors given our evidential history would have been permissible. If we have a high enough confidence in that, then we have some reason to fix the mistake. But given the obvious fact that human beings so often reason badly, it seems unlikely that my original priors were in fact permissible—if Bayesianism is objective, we should believe in the “original cognitive sin” of bad original priors. Perhaps, just as I speculated on strict objective Bayesianism, we have some reason to hope that our actual original priors were closer to permissible than any priors that would generate our new “mistaken” posteriors. Perhaps.
So every kind of Bayesian has some difficulties with what to do given a miscalculation. Objective Bayesians have some hope of having an answer, but only if they have some optimism in our actual original priors being not too far from permissibility.
It is interesting that the intuition that we should fix our “mistaken” posteriors leads to a rather “Catholic” view of things: although doubtless there is original cognitive sin in our original priors, these priors are sufficiently close to permissibility that cognitive repairs make rational sense. We have depravity of priors, but not total depravity.
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