Finding a green apple is, logically, evidence that all ravens are black. How can that be?
The statement 'all ravens are black' is logically equivalent to 'everything that is not black is not a raven.' Carl Hempel pointed out that if observing a black raven counts as evidence for the first statement, then by strict logic, observing any non-black, non-raven object, a green apple, a yellow pencil, should count as evidence for the second, and therefore for the first too. Can that really be right?
Reveal the answer
Hempel argued the logic is technically valid but the confirmation is vanishingly weak: a green apple confirms 'all ravens are black' only to an infinitesimal degree, since there are vastly more non-black things than ravens, so each non-raven observation barely moves the needle while an actual raven sighting moves it much further. Hempel published the paradox in 1945, and it remains a staple problem in the philosophy of scientific confirmation.
— Carl Gustav Hempel, Studies in the Logic of Confirmation — Mind, 1945