Suppose that your good friend, the police commissioner, tells you in strictest
confidence that the crime kingpin of your city is Wulky Wilkinsen. As a
rationalist, are you licensed to believe this statement? Put it this way: if you
go ahead and insult Wulky, I’d call you foolhardy. Since it is prudent to act as
if Wulky has a substantially higher-than-default probability of being a crime
boss, the police commissioner’s statement must have been strong Bayesian
evidence.
Our legal system will not imprison Wulky on the basis of the police
commissioner’s statement. It is not admissible as legal evidence. Maybe if you
locked up every person accused of being a crime boss by a police commissioner,
you’d initially catch a lot of crime bosses, and relatively few people the
commissioner just didn’t like. But unrestrained power attracts corruption like
honey attracts flies: over time, you’d catch fewer and fewer real crime bosses
(who would go to greater lengths to ensure anonymity), and more and more
innocent victims.
This does not mean that the police commissioner’s statement is not rational
evidence. It still has a lopsided likelihood ratio, and you’d still be a fool to
insult Wulky. But on a social level, in pursuit of a social goal, we
deliberately define “legal evidence” to include only particular kinds of
evidence, such as the police commissioner’s own observations on the night of
April 4th. All legal evidence should ideally be rational evidence, but not the
other way around. We impose special, strong, additional standards before we
anoint rational evidence as “legal evidence.”
As I write this sentence at 8:33 p.m., Pacific time, on August 18th, 2007, I am
wearing white socks. As a rationalist, are you licensed to believe the previous
statement? Yes. Could I testify to it in court? Yes. Is it a scientific
statement? No, because there is no experiment you can perform yourself to verify
it. Science is made up of generalizations which apply to many particular
instan
ayu-mushi
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