You know how I have that brain thing where I think about a million things all at once? Well, one of my favorite books is Girl, Interrupted. There's a passage in there that explains it really well, so I'm posting it here. Pay attention to the velocity part. (Yes, I'm keeping the paragraphs exactly as they are in the book, it's part of the style. Looks better single spaced with slight indentions to mark paragraphs.)

Velocity vs. Viscosity

Insanity comes in two basic varieties: slow and fast.

I'm not talking about onset or duration. I mean the quality of the insanity, the day-to-day business of being nuts.

There are a lot of names: depression, catatonia, mania, anxiety, agitation. They don't tell you much.

The predominant quality of the slow form is viscosity.

Experience is thick. Perceptions are thickened and dulled. Time is slow, dripping slowly through the clogged filter of thickened perception. The body temperature is low. The pulse is sluggish. The immune system is half-asleep. The organism is torpid and brackish. Even the reflexes are diminished, as if the lower leg couldn't be bothered to jerk itself out of its stupor when the knee is tapped.

Viscosity occurs on a cellular level. And so does velocity.

In contrast to viscosity's cellular coma, velocity endows every platelet and muscle fiber with a mind of its own, a means of knowing and commenting on its own behavior. There is too much perception, and beyond the plethora of perceptions, a plethora of thoughts about the perceptions and about the fact of having perceptions. Digestion could kill you! What I mean is the unceasing awareness of the processes of digestion could exhaust you to death. And digestion is just an involuntary sideline to thinking, which is where the real trouble beings.

Take a thought -- anything; it doesn't matter. I'm tired of sitting here in front of the nursing station: a perfectly reasonable thought. Here's what velocity does to it.

First, break down the sentence: I'm tired -- well, are you really tired, exactly? Is that like sleepy? You have to check all your body parts for sleepiness, and while you're doing that, there's a bombardment of images of sleepiness, along these lines: head falling onto pillow, head hitting pillow, Wynken, Blynken, and Nod, Little Nemo rubbing sleep from his eyes, a sea monster. Uh-oh, a sea monster. If you're lucky, you can avoid the sea monster and stick with sleepiness. Back to the pillow, memories of having mumps at age five, sensation of swollen cheeks on pillows and pain on salivation -- stop. Go back to sleepiness.

But the salivation notion is too alluring, and now there's an excursion into the mouth. You've been here before and it's bad. It's the tongue: Once you think of the tongue it becomes an intrustion. Why is the tongue so large? Why is it scratchy on the sides? Is that a vitamin deficiency? Could you remove the tongue? Wouldn't your mouth be less bothersome without it? There'd be more room in there. The tongue, now, every cell of the tongue, is enormous. It's a vast foreign object in your mouth.

Trying to diminish the size of your tongue, you focus your attention on its components: tip, smooth; back, bumpy; sides, scratchy, as noted earlier (vitamin deficiency); roots -- trouble. There are roots to the tongue. You've seen them, and if you put your finger in your mouth you can feel them, but you can't feel them with the tongue. It's a paradox.

Paradox. The tortoise and the hare. Achilles and the what? The tortoise? The tendon? The tongue?

Back to tongue. While you weren't thinking of it, it got a little smaller. But thinking of it makes it big again. Why is it scratchy on the sides? Is that a vitamin deficiency? You've thought these thoughts already, but now these thoughts have been stuck onto your tongue. They adhere to the existence of your tongue.

All of that took less than a minute, and there's still the rest of the sentence to figure out. And all you wanted, really, was to decide whether or not to stand up.

Viscosity and velocity are opposites, yet they can look the same. Viscosity causes the stillness of disinclination; velocity causes the stillness of fascination. An observer can't tell if a person is silent and still because inner life has stalled or because inner life is transfixingly busy.

Something common to both is repetitive thought. Experiences seem prerecorded, stylized. Particular patterns of thought get attached to particular movements or activities, and before you know it, it's impossible to approach that movement or activity without dislodging an avalanche of prethought thoughts.

A lethargic avalanche of synthetic thought can take days to fall. Part of the mute paralysis of viscosity comes from knowing every detail of what's ahead and having to wait for its arrival. Here comes the I'm-no-good thought. That takes care of today. All day the insistent dripping of I'm no good. The next thought, the next day, is I'm the Angel of Death. This thought has a glittering expanse of panic behind it, which is unreachable. Viscosity flattens the effervescence of panic.

These thoughts have no meaning. They are idiot mantras that exist in a prearranged cycle: I'm no good, I'm the Angel of Death, I'm stupid, I can't do anything. Thinking the first thought triggers the whole circuit. It's like the flu: first a sore throat, then, inevitably, a stuffy nose and a cough.

Once, these thoughts must have had a meaning. They must have meant what they said. But repetition has blunted them. They have become background music, a Muzak medley of self-hatred themes.

Which is worse, overload or underload? Luckily, I never had to choose. One or the other would assert itself, rush or dribble through me, and pass on.

Pass on to where? Back into my cells to lurk like a virus waiting for the next opportunity? Out into the ether of the world to wait for the circumstances that would provoke its reappearance? Endogenous or exogenous, nature or nurture -- it's the great myster of mental illness.