Depression, the best known of all mental issues—such that some still call it the "disease of the century"—is paradoxically still the most mysterious.

How to define depression? Where does it come from? Is it a neurological condition? Does it have genetic roots? Do our social and cultural environments play a part in its spreading? These and many more very of-the-moment questions remain unanswered and seem related to the same distress that has pained human beings, faced with the precarity of their existence, for centuries.