Logic

The Raven Paradox Is a Confusing Philosophical Conundrum

If you're trying to prove that all ravens are black, a black raven can obviously help your cause. The thing is, so can a green apple. Why?

The Raven Paradox

In all your life, have you ever seen a raven that wasn't black? That probably leads you to conclude that all ravens are black. Of course, it's impossible to see every raven that ever existed and ever will exist, but from the evidence you have on hand, it's pretty fair for you to make the statement, "All ravens are black." Put into a logical form, the argument looks like this:

  • Proposition 1: All ravens are black.
  • Evidence 1: This raven is black.

In the same way, scientists will make identical observations about a phenomenon — for example, when you drop a pencil it falls to the ground — and, with enough observations, make the conclusion that the phenomenon follows a natural law.

But according to logic, the statement "All ravens are black" has the equivalent form "All non-black things are non-ravens" — or in regular English, "Everything that isn't black isn't a raven." The same way that every black raven you see supports your first conclusion, every non-black non-raven you see (A green apple! A school bus! A Smurf!) supports it, too. This builds upon our argument like this:

  • Proposition 1: All ravens are black.
  • Proposition 2: All non-black things are non-ravens.
  • Evidence 1: This raven is black.
  • Evidence 2: This green apple is not a raven.

But doesn't it seem silly that literally any non-black thing you see could support your statement that all ravens are black?

This is the raven paradox, first introduced by the logician Carl Gustav Hempel in the 1940s. It's a seeming philosophical paradox that looks at how conclusions can be confirmed by positive instances. Why does a green apple support the statement "All ravens are black," when it has so little to do with ravens?

Is This Really a Paradox?

If so, it's a big deal. Though the raven example is almost absurdly simple, the paradox itself highlights a potential issue with the scientific method. Does every piece of evidence — even a piece of evidence unrelated to your topic (e.g. ravens) — really support your hypothesis, just because it doesn't contradict it? Is the scientific method taking us into fallacious territory?

Ravens have little bearing on our daily life, and their color even less, but a glitch in the scientific method would, indeed, be a big deal. The scientific method is used to test much higher-stakes hypotheses, like, you know, "This drug cures cancer" and "global temperatures are rising."

Ultimately, though, many (including Hempel himself) argue the raven paradox isn't so paradoxical. Though it doesn't jibe with our intuition that a green apple would have a bearing on raven's hue, that's a problem with our intuition. A green apple does provide an almost imperceptible grain of support for the "All ravens are black" hypothesis. A black raven just provides ... a lot more.

The Paradox of the Ravens

Can you follow what Hempel is saying?

Written by Curiosity Staff May 14, 2018

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