Everybody knows blue/green are correct categories, while grue/bleen are not.
Philosophers invented grue/bleen in order to be obviously incorrect categories, yet difficult to formally separate from the intuitively correct ones. There are of course less obvious cases, but the elucidation of the problem required them to come up with a particularly clear example.
Philosophers invented grue/bleen in order to be obviously incorrect categories, yet difficult to formally separate from the intuitively correct ones. There are of course less obvious cases, but the elucidation of the problem required them to come up with a particularly clear example.