The philosopher of science, Carl Hempel proposed this logical paradox concerning inductive evidence in the 1940's.
If you take the statement: 'all crows are black' - it is logically equivalent to the statement 'all non-black things are non-crows', so evidence for one theory should logically be
evidence for the other. Thus a yellow banana, being a non-black thing and a non-crow, should be accepted as a confirming example of the second statement, but
common-sense struggles to accept it as evidence for the first statement (that all crows are black)
In fact it is valid (if very slight) evidence, so long as it's non-blackness (yellowness) is recognised before it is tested for non-crow-ness – on the basis that
valid evidence must be able to potentially diminish the theory as well as support it. Something selected as a non-crow obviously cannot impact on the theory that
all crows are black no matter what colour it is.
Moncrieff is fascinated by this theory and seems intent on bringing it to bear on his current case.
However Sergeant Benskin finds it bewildering - how can nonsense about the blackness of crows have a real impact on the wave of art theft currently sweeping across
London?
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