Since there is no such thing as "mental health"--that can be defined by anyone anywhere in even remotely the same way, there is no such thing as mental illness. There is only a spectrum of behaviors that are judged by society.

Once, society judged that marriage between different races was wrong. It then evolved and realized IT was wrong, rather than the marriages. The same process is underway for homosexual marriages.

There are people who are obviously mentally ill, but no one, anywhere, who is obviously mentally healthy.

It’s a social construct. Both are.

It’s also a personal construct: “I have determined that I am mentally ill. Please alleviate me of much of my accountability and give me drugs.”

Psychology, as a business, says “OK!”

ka-CHING!

No one knows. It’s like the argument between atheists and theists: absolutely fucking pointless.