German philosopher Carl Gustav Hempel introduced this in the 1940s, questioning what composes evidence for a hypothesis. Here is how the paradox works:
You see a raven, and it is back. Then you see more ravens, and they all tend to be black. These observations prompt the scientist in you to form the hypothesis that all ravens are back.
So far, so good. Now it becomes a conditional (statement of the form: “if A, then B”). As per logic, a conditional is equivalent to its contrapositive:
If A, then B == If not B, then not A
For ravens, the equivalent statement is:
All ravens are black == All non-back things are non-raven, or if an object is not black, then it is not raven
Now let’s collect evidence for the hypothesis. Every black raven is a piece of evidence. Every non-black non-raven also has to be evidence! Green grass, red shirts, and yellow flowers are some examples.
So, what is the issue with this? Well, there could be rare non-black ravens which escape our sight. In the original form of the hypothesis (conditional), you only sample ravens and verify their colour. But in the contrapositive form, you can potentially collect an infinite number of objects (every non-black entity in the world) and appear to strengthen your hypothesis significantly.
