First of all, let's look at the two qualities of mind you're trying to avoid while engaging in shamatha meditation:
* Laxity: this typically includes stupor (ranging from a mild zoning out to effectively vegetating) and torpor (being slightly drowsy to falling asleep).
* Excitation: letting "discursive thoughts" take control. It's never elaborated in Buddhist texts what is meant by "discursive", but of the two main definitions, the former seems more applicable:
1. Passing from one topic to another; ranging over a wide field; digressive; rambling.There are also descriptions urging you not to attempt to have an empty mind, which they associate with vegetating. At first, all these seem eminently reasonable. But eventually you discover things that seem to clash with those descriptions:
2. Utilizing, marked by, or based on analytical reasoning -- contrasted with intuitive.
* In the highly advanced dream yoga, you learn to fall asleep while vividly conscious, and maintain your awareness through sleep. Crazy as it sounds, I can assure you it's possible.
* If meditation made you unable to pass from one topic to another in thought, who would want to do it? Maybe they mean you shouldn't do that while meditating, but this is also wrong: in the most advanced form of practice, you are completely aware of "rambling thought" if it happens to arise (which happens much less frequently).
* Eventually your mind does in fact become "empty" in some sense. In fact, it is claimed in Tibetan Buddhism that the ultimate nature of mind (and reality) is emptiness!
Okay so now for the intuition. Think of how the following situations feel.
* You're meditating on the breath, but at some point notice yourself thinking about something else. In fact, you've been off it for thirty seconds without noticing.
* You're reading a textbook for school, but have to re-read paragraphs after realizing that the words were going in one eye and out the other.
* You're sitting in the waiting room at the doctor's office and suddenly notice that you don't know where the last ten minutes went. It's just as well, you think, because it's boring in there.
Each of these is a description of zoning out. In some cases, your mind may be running furiously, churning up one useless (but deliciously interesting) thought after another; in other cases, you're totally blank.
Classically these are distinguished as stupor and excitation, but in actuality they're fundamentally the same. As for torpor, it seems problematic only in that it seems to induce spaciness no matter how hard we try to fight it.
So we've reduced the faults to just one that we're intimately familiar with: being zoned out. The title of this post refers to an analogy I like to make: presence of mind is like having the clutch engaged in your vehicle (for those who are unfamiliar with how cars work, the clutch is a mechanism that transfers power from the engine to the transmission, and ultimately, the wheels). When the clutch is disengaged, it makes no difference if the engine is spinning wildly (excitation) or totally at rest (stupor); the end result is the same: no power to the wheels.
The goal of "mindfulness" meditation is to keep the clutch continuously engaged. By doing so, we become better at reading, listening, noticing our emotions, ... or in short, living life. So get a feel for it, even if you don't like formal meditation. I suspect you'll find yourself "out of it" more often than you normally realize, and the little extra mindfulness this exercise automatically brings you will surely be worth the effort.