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Hempel's Ravens


Hempel's Ravens

Ship of Theseus

1=0 & 1=2

Logical Paradoxes

Optical Illusions

"Suppose an ornithologist wishes to determine whether or not all ravens are black. The reasonable thing for him to do is to go outside and look for ravens. If he finds even one that isn't black, that proves that not all ravens are black. If, on the other hand, he sees thousands of ravens and every single one of them is black, then that offers support for the proposition that they are all in fact black. Although no amount of observations can ever conclusively prove the hypothesis, each new black raven found provides additional evidence for it.

But now suppose that our ornithologist, after seeing thousands of ravens, becomes tired of looking for them, and decides to try a different method. He reasons as follows:

The statement "all ravens are black" is logically equivalent to the statement "all nonblack objects are nonravens". When you see a blue sky, a yellow submarine, or any other nonblack nonraven, that supports the proposition that all nonblack objects are nonravens. But in that case, it also supports the proposition that all ravens are black. So all one has to do is look around at ordinary objects to acquire evidence that all ravens are black! No need to go out in the woods in search of ravens, since each nonblack nonraven is also evidence for that hypothesis. This is the paradox known as Hempel's ravens, named after Carl Hempel, who discovered it in 1946.



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