He Hath Ever But Slenderly Known Himself

From Andrew Solomon's The Noonday Demon: An Atlas of Depression:

It is arguably the case that depressed people have a more accurate view of the world around them than do nondepressed people. Those who perceive themselves to be not much liked are probably closer to the mark than those who believe that they enjoy universal love. A depressive may have better judgement than a healthy person. Studies have shown depressed and nondepressed people are equally good at answering abstract questions. When asked, however, about their control over an event, nondepressed people invariably believe themselves to have more control than they really have, and depressed people give an accurate assessment. In a study done with a video game, depressed people who played for half an hour knew just how many little monsters they had killed; the undepressed people guessed four to six times more than they had actually hit. Freud observed that the melancholic has "a keener eye for the truth than others who are not melancholic." Perfectly accurate understanding of the world and the self was not an evolutionary priority; it did not serve the purpose of species preservation. Too optimistic a view results in foolish risk-taking, but moderate optimism is a strong selective advantage. "Normal human thought and perception." wrote Shelley E. Taylor in her recent, startling Positive Illusions, "is marked not by accuracy but positive self-enhancing illusions about the self, the world, and the future. Moreover, these illusions appear actually to be adaptive, promoting rather than undermining mental health. . . . The mildly depressed appear to have more accurate views of themselves, the world, and the future than do normal people . . . [they] clearly lack the illusions that in normal people promote mental health and buffer them against setbacks." 

The fact of the matter is that existentialism is as true as depressiveness. Life is futile. We cannot know why we are here. Love is always imperfect. The isolation of bodily individuality can never be breached. No matter what you do on this earth, you will die. It is a selective advantage to be able to tolerate these things, and to go on--to strive, to seek, to find, and not to yield. [...] Depressives have seen the world too clearly, have lost the selective advantage of blindness. (433-434)

While I understand the research is not quite so clear as all that, and while depression is capable of speaking lies, it is also the unfortunate case that depression sometimes tells the truth. Those with depression know in their bones that their automatic negative thoughts are true, and part of the practice of therapy is to learn to shut those thoughts out, but there's an equivocation there: do you ignore the thoughts, or deny them? Because, if we're going to be honest, some of the negative thoughts are true; the person with depression is right, and they know it. This makes it so much harder to identify the negative thoughts which are just lies your depression is telling you.

The fall-back position, I guess, is interpretation; change the metric and you can change the answer. I can say for sure that positive self-talk works (sometimes), but it's hard not to believe that positive self-talk is a ritual of lying to yourself. Maybe mental health is constituted by having more rather than less productive delusions.