we argue that depression is in fact an adaptation, a state of mind which brings real costs, but also brings real benefits.

While I do believe there is some merit to this theory. Some of the symptoms that go along with depression can lead to problems resolution, such as rumination that causes a person to think intensely about the problem that they are facing. I believe that every thing that we do is adaptive and supports our growth. When that adaptive process gets stuck it becomes maladaptive. Rumination is disabilitating when there is the lack of energy to follow though with the implementation of the solutions. Also with depression individuals are not aware of the problem that brought the depression on or they have been depressed so long that their thinking patters do not return too normal.