I must respectfully disagree with the Boston reader who asserts that Bartleby is "a case study in clinical depression." That's definitely one way to interpret the story, but it defuses a great deal of the tale's power. If we believe that Bartleby is simply a victim of mental illness, we might begin to believe that if only doctors had Prozac in the 19th Century, poor old Bartleby would have chippered up and gone home dancing.
Bartleby's refusal, his famous "I prefer not to," seems more like a deliberate and sane NO. People did know what depression was back then (though it was generally called melancholia, instead). Bartleby's condition (our condition?) is something much deeper, much more terrifying-- the possibility that one can observe the world from a completely rational mind and decide that participation is not worth it.
If we decide that Bartleby's problem was depression, must we call Kurtz a paranoid schizophrenic? All of Beckett's characters could use a Xanax prescription, because they seem pretty bleak, too.
Bartleby is fascinating because of what we don't know; Melville is the great American exploiter of Negative Capability, Keats's term, defined as "when a man is capable of being in uncertainties, mysteries, doubts, without any irritable reaching after fact and reason."
It is because Melville is willing to refrain from that "irritable reaching after fact and reason" that Bartleby (and Benito Cereno and Moby-Dick) is a story that lingers in the imagination. If we knew why the man simply quit the business of life, if we knew it was a deficiency of chemicals in his brain or whatever, we would not be so haunted by his fate.