I think of depression as operating on two levels. There is the interior level, where depression is an emotional state - a set of feelings that are negative and painful. Then there is the functional or behavioral level, where a person's capacity for action seems to be reduced, so that it's hard to work toward goals . . . hard to get stuff accomplished. I think I sense what you are referring to. I have had spells where I didn't feel particularly bad, but I seemed to have little interest in getting anything done. I didn't feel "depressed" emotionally, but I sure wasn't functioning at my normal capacity. IMHO, that's a state of being depressed.

I'ld go further and say that, with chronic depression, there can be spans of time where you don't feel very despondent/sad and you manage to tend to your resonsibilities and do the daily work life requires of you, but you get little real satisfaction out of living. I'ld call that being in a state of depression. We've all heard of people who were going along, seeming okay and performing competently at work, who committed suicide. They may have been recently seen in social situations displaying nothing abnormal moodwise. But something afflicted the person that made the individual vulnerable to completely giving up. The affliction must have been quite severe. When an individual starts experiencing depression early in life, I think they are apt to adjust to it, so that they can function fairly normally in a depressed state. People who are not chronically depressed have no real sense of what that is like.

Humans don't require continuous joy. They do need to have memories of joy and a reasonable expectation of there being some joy in the future. Otherwise, life hardly seems worth the effort. Animal behavior is mostly instinctual. Everyday the deer looks for grass to graze on, and the bird looks for seeds to crack open and swallow. Their behavior isn't mood-driven. That starts to change as animal intelligence increases. Some animal experts say that dolphins can experience mental depression and even commit suicide. They claim a severely-despondent dolphin will submerge and simply not bother coming to the water's surface to breathe. In that way, it escapes the distress of its existence. Intelligent creatures don't merely want to survive. They want to derive some satisfaction out of life . . . some joy.