In some discussions of the logic of induction, one runs into a paradox of the ravens, otherwise known as Hempel's paradox, in recognition of philosopher Carl Gustav Hempel. I'd like to run through it now, if only in an attempt to keep my mind nimble.
Hempel asks us to consider the hypothesis (1) all ravens are black. Now, we naturally think that if we can justify this proposition as a matter of induction, we can do so by looking at a lot of ravens, to see if all of them that we can find are black.
But we surely don't look at apples, to see if they are some color other than black (say, red or green). Because we implicitly but strongly believe (2) observations of non-ravens are not relevant to verifying a generalization about the look of ravens.
But ... Hempel's paradox challenges (2). After all, (1) would seem to be the logical equivalent of (3) All non-black items are non-ravens.
So: whenever I see a green apple, I see a non-black item that is a non-raven. Why do we revolt at the idea of treating this as an observation tending to support (1)?
There is a lot that one might say about this. I suspect that if I understood Bayesian probability theory better I would have a ready response.
One might, though, simply say, "nevermore."