Solitude and isolation. Dwelling on problems for long periods of time. Not being satisfied with life as usual. Preferring to think before (or instead of) acting. Taking a long time to complete a seemingly simple task, because you can see the full ramifications of the task and how difficult it really is. Concern with the abstract and the illusionary/visionary instead of the immediate. Some of the attitude towards focus and concentration, although that varies.

In short, I think that depression can be somewhat characterized as a sensitivity to, or inability to filter out, certain perceptions, especially abstract concerns. What the author is advocating seems to be a conscious effort to seek those perceptions. My opinions are, of course, biased by my own experiences.