I saw an old quote saying "My friends are mankind." Is this correct? The word "are" is generally used with multiple objects, so it works properly with friends - ie, "My friends are great" is correct. But "mankind" is singular - saying "Mankind are great" would definitely be incorrect.
Would "My friends are mankind" be proper grammar, or would it need to be changed to "my friends are the whole of mankind" in order to change mankind into a plural?