Oh my, there is ASTOUNDING significance in Bartleby's frequent quotation of "I would prefer not to" in that dear Bartleby is exemplifying the anti-Transcendentalist viewpoint of Melville. In this way, Bartleby knocks order upside down through his continual, anti-Transcendentalist request.
Let's look at the progression into chaos due to upsetting the natural order. At first, Bartleby seems to be an adequate scrivener. He works hard until one day he refuses to copy one document by using his famous line ("I would prefer not to"). This escalates into Bartleby not "preferring" to do ANY of his work. Further, Bartleby is now fully freeloading at the lawyer's office by living there full-time and eating only Ginger Nut's cakes and apples.
My ﬁrst emotions had been those of pure melancholy and sincerest pity; but just in proportion as the forlornness of Bartleby grew and grew to my imagination, did that same melancholy merge into fear, that pity into repulsion.
It is interesting here how the lawyer "begs" Bartleby to work or to leave. Even this is an upset of who is in control. (Melville, by the way, loves that upset. Think of Moby Dick where the power of the novel is the white whale driven by brute instinct.) This is a very anti-Transcendentalist idea. In this way, Bartleby (and not his employer) is in complete control of the situation! The lawyer even MOVES HIS BUSINESS to escape Bartleby's doings! It is only the new owner who has the gumption to evict Bartleby who simply stays in the building continuing to frustrate the lawyer who offers to take him home with him. Finally, the narrator leaves the city out of complete frustration.
Bartleby defies a new order when he enters the city jail (the Tombs). Because he is requested to eat, he replies with his usual, "I would prefer not to." Now Bartleby is not just denying the hierarchical order (even with the lawyer still trying to help him), but the NATURAL order. Organisms must all eat in order to live. This doesn't matter due to anti-Transcendentalist ideals. Due to the lack of food (in addition to the depressing conditions of the prison), Bartleby dies.
Is this mental illness? Simple introversion? Depression? It doesn't really matter. Mental illness IN ITSELF is a denial of the natural orders of the brain! (An anti-Transcendentalist's paradise! Think of the obsession of Captain Ahab!) Regardless, Melville ends with what eNotes calls the "pathetic universality" of the last line: “Ah, Bartleby! Ah, humanity!” There can be nothing more anti-Transcendentalist than the death of a human by defying the natural order of things. Mental illness or not, in defying nature, Bartleby hastens death.