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Privileging the Hypothesis

57 Eliezer_Yudkowsky 29 September 2009 12:40AM

Suppose that the police of Largeville, a town with a million inhabitants, are investigating a murder in which there are few or no clues—the victim was stabbed to death in an alley, and there are no fingerprints and no witnesses.

Then, one of the detectives says, "Well... we have no idea who did it... no particular evidence singling out any of the million people in this city... but let's consider the hypothesis that this murder was committed by Mortimer Q. Snodgrass, who lives at 128 Ordinary Ln.  It could have been him, after all."

I'll label this the fallacy of privileging the hypothesis.  (Do let me know if it already has an official name—I can't recall seeing it described.)

Now the detective may perhaps have some form of rational evidence which is not legal evidence admissible in court—hearsay from an informant, for example.  But if the detective does not have some justification already in hand for promoting Mortimer to the police's special attention—if the name is pulled entirely out of a hat—then Mortimer's rights are being violated.

And this is true even if the detective is not claiming that Mortimer "did" do it, but only asking the police to spend time pondering that Mortimer might have done it—unjustifiably promoting that particular hypothesis to attention.  It's human nature to look for confirmation rather than disconfirmation.  Suppose that three detectives each suggest their hated enemies, as names to be considered; and Mortimer is brown-haired, Frederick is black-haired, and Helen is blonde.  Then a witness is found who says that the person leaving the scene was brown-haired.  "Aha!" say the police.  "We previously had no evidence to distinguish among the possibilities, but now we know that Mortimer did it!"

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