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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.

"The Myth of Mental Illness" by Thomas Szasz in "American Psycholigist", Volume 15 (pp. 113-118), psychclassics.yorku.ca. 1960.
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.