Raven’s Paradox

How Can the Existence of a White House Prove Ravens Exist?

In the mid 1960’s Carl Hempel introduced the Raven’s Paradox. This thought experiment was designed to show the problem of “the logic of support”. Simply put the argument runs as follows:

1. All ravens are black

2. If all ravens are black, then all non-black things are non-ravens

This seems sound. If ravens must be black, if it a necessary condition of their existence, then something which does not have this quality cannot be a raven. Statement 2. is said to have logical equivalence with Statement 1 – this means they are saying “the same thing in different terms”.

But, from this sound piece of reasoning we can use seemingly irrelevant observations to prove the argument. For example we could say:

3. The man’s house is white

Statement 3. is totally irrelevant yet satisfies the logic of statement 2. (i.e. it is neither a raven, nor is it black). And because statement 2. is equivalent to Statement.1, logically, it supports this statement too… In effect the problem can be summarised as follows:

Of course, intuitively, this seems wrong. Indeed, while Hempel actually accepted the conclusion but many others haven’t.

Some people focus on the idea of a raven and it’s blackness (“What if we saw a white raven?”), rather than the logical fallacy. I think this misses the point: in biology there will always be exceptions (black swans). Perhaps it is better to focus on something that has a more necessary relationship with its properties, a triangle and it’s three sides for example. It is impossible to think of a triangle without three sides, so you could re-run the problem with this statement and its logical equivalent.

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Raven’s Paradox

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