Doubt is to certainty as neurosis is to psychosis. The neurotic is in doubt and has fears about persons and things; the psychotic has convictions and makes claims about them. In short, the neurotic has problems, the psychotic has solutions. ↗
Mental illness, of course, is not literally a 'thing' - or physical object - and hence it can 'exist' only in the same sort of way in which other theoretical concepts exist. ↗