A green apple is, technically, evidence that all ravens are black.
This is certainly no laughing matter. Philosophers and mathematicians have debated the issue since the 1940s, when Carl Gustav Hempel, a philosopher, first proposed it.
The Trap Hidden in Logic
Consider the scenario where you wish to evaluate the hypothesis that all ravens are black. You venture out to spot ravens. Every black raven you spot adds a touch of confidence. It does not feel strange at all. That's essentially the process of collecting evidence.
The statement "all ravens are black" is equivalent, in formal terms, to "all non-black things are non-ravens." The claims are not separate. The statement is identical, presented from different angles. In logic, one is called the contrapositive of the other, and a statement and its contrapositive are always both true or both false. You can't have one without the other.
Therefore, if observing a black raven validates the first claim, then sighting any non-black non-raven should equally validate the second. So, since the second is identical to the first, it must also support the first. Your green apple isn't black, and it's not a raven either. So does the blue sky. The brown coffee in your cup. Every non-black object near you, technically speaking, is silently gathering as evidence.
The Comfortable Lie
The most popular response to this problem accepts the conclusion but argues that the evidence from each apple is so small it barely registers. Non-black objects vastly outnumber ravens, thus each non-black non-raven makes a negligible contribution. In theory, an apple serves as evidence, but in practice, it holds little significance. Most people find this answer and feel relieved, and move on.
But it might be too early to feel relieved. Several thinkers have countered, asserting that the response merely skirts the fundamental issue without addressing it properly. The measure of evidence isn't what matters. The core question is whether the apple counts as evidence and, if so, under what circumstances.
It's Not the Apple. It's You.
Whether something that isn't black and isn't a raven can be considered evidence actually hinges entirely on how it was observed. Walk outside and notice a red apple sitting on the ground. And that apple tells you nothing about ravens. Apples always appear naturally, regardless of your testing activities. Suppose you were systematically verifying non-black objects, examining each one sequentially. The question for each: Could this be a raven? If you check a thousand non-black things and none of them turn out to be ravens, that actually supports the hypothesis. Not because the objects are different. The apple stays an apple. What altered was the procedure that generated the observation.
Evidence requires more than just observations. It is the observation and the process that produced it. The same apple, in the same world, might be considered or not considered based on your perspective.
Nobody Won. Nobody Will.
No universal solution has been accepted for this paradox. Some thinkers argue that the problem reveals a genuine flaw in how logic handles inductive reasoning. Others believe it was never intended to have a solution. Hempel outlined that broad generalizations are more vulnerable than they appear, and that the scientific process requires external pressure.
The exact same logical framework that validates "all ravens are black" can also generate an equivalent level of confidence for its complete opposite. A scientist who dedicated their life solely to studying white ravens would conclude, through the same logic, that every raven is white. Every non-white non-raven they had come across would increase that belief. The logic is sound and impeccable. The conclusion would be wrong.
There's no debate over the apple. The core debate invariably centered on what constitutes a fair test.