If two statements are logically equivalent, if they assert exactly the same thing, then any evidence for one is evidence for the other. This principle appears to be truism; the Hempelís Ravens paradox, however, seems to show that itís false.
Using this principle, the Hempelís Ravens paradox suggests that an observation of a green parrot is evidence that ravens are black. The paradox goes like this:
Consider the two statements:
(1) All ravens are black.
(2) Everything that isn't black, isn't a raven.
These two statements say exactly the same thing. The first statement says that everything of a particular kind has a certain property. The second statement says that everything that lacks that property isnít of that kind.
The two statements are therefore logically equivalent; they are true and false in exactly the same circumstances. If there is anything that is a raven but isnít black then both (1) and (2) are false; oherwise, they are both true. As the two statements are logically equivalent, any observation that supports one will also support the other.
Suppose, then, that I observe a green parrot. This observation confirms (2), "Everything that isn't black isn't a raven". A green parrot isnít black and isnít a raven. The observation is evidence that (2) is true.
Given what has been said so far, my observation of a green parrot must also confirm (1). (1) and (2) are logically equivalent, so any evidence for one is evidence for the other. My observation of a green parrot, then, is evidence for the statement, ďAll ravens are blackĒ. In fact, any observation of something that isnít black and isnít a raven is evidence that ravens are black.
This, though, is absurd; there is no way that we can discover what color a raven is without looking at a raven.