Noun
fixed idea (plural fixed ideas)
An idée fixe, an obsession.
My second fixed idea is the uselessness of men above sixty years of age, and the incalculable benefit it would be in commercial, political, and in professional life, if as a matter of course, men stopped work at this age. William Osler
That faith makes blessed under certain circumstances, that blessedness does not make of a fixed idea a true idea, that faith moves no mountains but puts mountains where there are none: a quick walk through a madhouse enlightens one sufficiently about this. Friedrich Nietzsche