Great question. I think a "mental illness" is exactly like a physical illness. I hope that modern medicine continues to improve so that we can "see" and treat mental illness just like we can any other illness like diabetes or cancer. I was discussing with a group the other day how difficult it is to encounter those types of people who disregard the seriousness of bipolar, depression, BPD, etc. It is disheartening to say the least. One person said that they would rather actually have one of the above mentioned physical illnesses instead of anxiety. She meant no offense by this, it was just out of frustration and hopelessness. There are diagnostic criteria that define exactly what a mental illness is, but the fact that we are even having this discussion shows that those criteria don't give us the complete understanding we want and need.