@Nessen well if you are trying to tell a person whether they are "dumb" or "smart", I'd say you can't since there isn't any CLEAR indicator about what makes a person "dumb" or "smart". Then, lets make one, let us say the indicator is IQ, and IQ below 80 is considered dumb (please, just for the sake of the argument, I don't even know what the average is). That way, we can now clearly tell the person whether they are "dumb" or "smart" based on the IQ they have.
Now, apply that kind of logic to "mentally ill" and the opposite of it.
also, I think it is kind of ignorant to say something like "mental illness exists in the first place, no?" when there is a branch of science called psychology, which (probably a fair amount of it, dunno) covers topic as mental illness.