What exactly is melancholy, and what (if anything) should we do about it?

If you are a living, breathing human being, chances are you have felt sad at least a few times in your life. But what exactly is melancholy, and what (if anything) should we do about it? Courtney Stephens details our still-evolving understanding of sadness -- and even makes a case for its usefulness.

http://ed.ted.com/lessons/a-brief-history-of-melancholy-courtney-stephens​