Is there such a thing as "high-functioning depression"?

As far as I can tell, misery and inertia/paralysis are semi-independent factors.

First, look into http://en.wikipedia.org/wiki/Dysthymia . Second, large parts of my family consider emself to be hardwired to be just not very happy, on the left side of the hard-wired-to-happyness-bellcurve. We pretty much resigned to it. Third, are you sure, by written review of your mental states, that your thoughts are consistent? There can be really really subtle differences; when I review my mental state I need to use more criteria than reasonable/unreasonable.

Have you considered that you might be partially depressed all the time?

For some value of 'thinking', it can, though I suspect that most people can't easily do the thing I have in mind.

During my last round of depression, about two and a half years ago, I had a brief period of suicidal ideation - the first time I'd experienced such a thing. I recognized it for what it was, assumed it would be temporary, and basically responded with variations on 'so what?' from there: Yes, I want to walk in front of this bus. So what? That's not what I'm going to do, so it doesn't matter that that's the case. (This is very similar to how "even if killing babies is the right thing to do, it's still the wrong thing to do".)

I suspect that fighting with the part of my brain that was generating those thoughts - trying to get it to stop doing so - would have been counterproductive. Instead, I took it as a valuable communication that things really had reached emergency status, got myself out of the situation that was causing the problem (yes, I was lucky to be in a situation where that was a relatively easily implemented option), and was fine.

The elephant-and-rider metaphor is relevant here. Just because the elephant is panicking and trying to roll over or run away doesn't mean that the best answer is to beat it.

It's almost like having two consistent, rational versions of yourself and switching between them. (I say "almost" because it's more like a gradient.) When you're in the depressed state, you have no access to the functional state. When depressed I tend to feel hyper-rational and gravitate towards ideas that allow me to dismiss other aspects of my life as illusory.

What scientism said is congruent with my experiences. I tried one time while being on the strongly-irrational/strongly-depressed-side to fight the irrationality. I refused the negative emotions to be accurate and trained myself to flag and consciouly counter all irrational thoughts with the accurate, positive thoughts. I described it then as a civil war inside my body. It was highly exhausting, my body and brain ached as if overworked, hands trembled, and I was highly agitated. I reported it then as a really good feeling, but the depression was lurking in the depths and attacking every few minutes or so with a "depressive" thought. It was like a two-day episode of being manic.

Since then I did not investigate its reproduceability, it was rather frightening, costed a lot of willpower and, luckily, I have not been so deep into depression again.