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I am interested in the idea that mood fluctuation provides a more complete view or perspective. In class we discussed the how different moods allow us to perceive a cat, for example, differently depending on mood. This idea makes sense to me as it gives some explanation as to why moods evolved. In a larger sense, I think awareness of the benefits of mood fluctuation can help us become better adjusted individuals. If we understand that sadness has benefits, we can become more appreciative of a sad mood and of mood fluctuations in general. 

 I think it is completely true that mood affects our perspective, even in the most basic sense. If you were to ask a sad person if a glass was half full or half empty, their mood would most likely literally change the way they saw the cup. Based on this, I think the simplest way to define depression, is as chronic intensely sad mood. In other words, as a result of the lack of mood fluctuation, a depressed person can only see the "sad" perspective.