me_irl

Well what is a mental illness? Historically you had an illness if something caused you to not function properly at the things society expected you to function in. You can't remember things clearly anymore and it affects your career or social life? You now are considered to have the mental illness of dementia. Are you just slightly forgetful but can still function fine? You are not ill, instead that forgetfulness is just a character trait of yours. And it goes on like this. Back in the day the expectations to marry the opposite sex and have children was huge so being gay was considered a mental illness. Today it's more accepted and being gay is just part of who you are (in some places at least). And it goes on like this.

I think not much has changed in this regard. If a certain mental affliction doesn't affect your social or work life in a meaningful way it's not really considered an illness. So if you take this as a definition of being mentally healthy there are certainly lots of people out there who have no affliction which seriously hurt their current social or work life. However this of course doesn't mean that these people are completely free from any mental affliction, it just means these mental afflictions are not considered serious in today's society.

And these things change all the time as society's expectations change. Back in the day when only a small minority could read and write there was not a diagnosis for dyslexia. Now that reading ability is expected of everyone in today's society we have defined dyslexia. As new societal expectations come and go the list of diagnosable illnesses will certainly have things getting removed and added. Let's say theoretically some people can't handle zero gravity, today this is not relevant for the vast majority of people so there is not any diagnosis for such an illness. But if more and more people are expected to live in zero G then such a diagnosis would be sure to be defined if such an affliction exists.

Is this definition reasonable? Well that's a completely different question. But as I see it this is how it is generally defined right now.