Cats

The world is full of cats.

Along comes Fred, who offers a definition of "cat" such that nothing previously identified as a cat is a cat.

And then all the biologists accept Fred's definition and commence to explain to everyone that there aren't really any cats, it's all a figment of everybody's imagination.

And if you look at them funny and keep on calling "cats" and regarding as cats the things everybody has always regarded as cats, they become very annoyed.

For "cats" put "races".