Apparently seeing green apples is proof that all ravens are black

Spotting a green apple supports the idea that all ravens are black. It sounds like an absurd claim, but consider logician Carl Gustav Hempel's argument, which he published in January 1945 issue of the journal Mind.

"All ravens are black," is a hypothesis that's equivalent to saying, "If something is not black, then it is not a raven." So, by seeing a green apple (which is neither black nor a raven), you're gathering evidence that supports the hypothesis that all ravens are black.

Intuitively, a green apple seems irrelevant to black ravens, and Hempel himself acknowledged this paradox. We know apples aren't ravens, so their color seems unrelated. But in strict logical terms, each non-black non-raven does incrementally support the hypothesis.

However, while a green apple might technically be evidence under Hempel's framework, its contribution is so negligible that it's practically irrelevant. Observing thousands of black ravens is far more convincing than noting a myriad of non-black non-ravens.

But… what if you encountered a black apple? Does that support or weaken the hypothesis that all ravens are black?

• The paradox of The Bottle Imp
• The coin paradox
• Why not take a moment to ponder the Fermi Paradox?