“[If there was] certainty that an acute episode [of depression] will last only a week, a month, even a year, it would change everything. It would still be a ghastly ordeal, but the worst thing about it — the incessant yearning for death, the compulsion toward suicide — would drop away. But no, a limited depression, a depression with hope, is a contradiction. The experience of convulsive pain, along with the conviction that it will never end except in death — that is the definition of a severe depression.”