It's not a point of whether or not it's supposed to be valid, but whether or not the creator can maintain _exclusive_ ownership of the copyrighted material. If

It's not a point of whether or not it's supposed to be valid, but whether or not the creator can maintain _exclusive_ ownership of the copyrighted material. If someone wrote a math book 100 years ago, that math book will still be correct today, even if it's in the public domain.