My take on depression is that it's an internal fight between an emotion (usually fear, anger or guilt) and a thought that says we shouldn't be feeling it (because there's no logical reason for it, etc.). The conflict gets so intense that the whole emotional system shuts down resulting in full on apathy, which is the cause of most of the symptoms of depression.

Apathy, in my case, felt like the flu. I didn't have energy, didn't want to do anything, felt weak and soulless.

This is not the official definition, but it's a workable description that also points to a solution - process the apathy and then the underlying emotions and the depression should lift. That's what the app I built helps with. But as usual, YMMV.