He excludes the only examples we have, which is fine for his purposes, though I’m skeptical it’s useful as a definition, especially since “some difference” is an unclear and easily moved bar. However, it doesn’t change the way we want to do prediction about whether something different is possible. That is, even if the example is excluded, it is very relevant for the question “is something in the class possible to specify.”
He excludes the only examples we have, which is fine for his purposes, though I’m skeptical it’s useful as a definition, especially since “some difference” is an unclear and easily moved bar. However, it doesn’t change the way we want to do prediction about whether something different is possible. That is, even if the example is excluded, it is very relevant for the question “is something in the class possible to specify.”