The fact it has survived so long - and not been eradicated by evolution - indicates it has helped the human race become stronger.
"I have received e-mails from ex-sufferers saying in retrospect it probably did help them because they changed direction, a new career for example, and as a result they're more content day-to-day than before the depression."
Aristotle believed depression to be of great value because of the insights it could bring. There is also an increased empathy in people who have or have had depression, he says, because they become more attuned to other people's suffering.
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