Distractions in Meditation

Is it possible that, despite the absence of thoughts, something other than arising cognitions pulls my attention away from the meditation object?

Yes, of course. If you are attempting to focus on something, any other experience can pull your attention away.

Just to be clear: I am asking what specifically are the type of possible distractions which may arise in meditation.

Sights, sounds, tastes, feelings, smells, thoughts.

I'm also wondering what is the relative strength of these distractions, for example: does a single thought break concentration? Does paying attention to some external object merely once break concentration?

The strength of a distraction entirely depends on how much time you devote to observing it. If you hear a sound and focus on it for only a moment, it was a small break in concentration. If you smell something and keep focusing on it over and over, it is a large break in concentration.

If your intention is to cultivate concentration skills, keep doing what you are doing. You sound as if you have extremely good concentration skills already and are now grappling with only very small breaks in concentration.

If your intention is to cultivate wisdom and see the truth, you have already overcome one of the largest hurdles, quieting the mind. Now it is time to start observing all experience, and not focusing on a meditation object.

Cultivating concentration is one method of learning to quiet the mind. Once you have achieved that, on the path to wisdom, there is no more benefit to enhancing concentration.

As you most likely already know, when we think, there is an automatic process of identification and labeling that occurs. When you hear a sound, you immediately think "that is a bird". This is why we always meditate with our eyes shut, for we see/identify/label "different objects" in our sight without even thinking about it. By cultivating concentration, we learn to quiet these thoughts.

You sound like a very advanced meditation practitioner that is extremely skilled in concentration! If I were you, I would stop concentrating and without thinking observe experience. While you do that, try to find answers to questions like:

What is I? What is not I? What is internal? What is external? What is experience? How far away from you does experience occur? How close to you does experience occur? What is physical? What is non-physical?

Thoughts and preconceptions mask true experience. Do not believe you need to have unbroken concentration for X amount of time to advance knowledge and gain wisdom. If you rarely ever have thoughts during your practice, you already have the tools you need. You have done a great job, well done! I wish you well on your path and hope you get where you'd like to be going soon. Forgive me if I made too many assumptions of your intentions. =]

Entering access concentration and then leaving access concentration can pull you away from the meditation object. 'Pull' is probably the wrong word here. It's more of a transition from one state to another; a higher absorption where, in some cases, bodily phenomena is left to fall away and this can include the meditation object. This is sometimes termed entering a jhana. You may have just fallen backwards though but it's helpful to recognise that you may be developing here. Regular visitations and experience will give you more awareness about what may be happening for you.

Alternativly, loss of the object may be related to one of the hindrances...

The Five Hindrances

Does a single thought break concentration?

Concentration can be broken by the degree of attention you put into that thought.

Does paying attention to some external object merely once break concentration?

So, for instance, there is a sound... but the sound is being interpreted by your body. Watch how your mind and body responds to the sound. Concentration can be broken by investing your attention into the thought about the sound. Thought will arise. Observe thought with curiosity. After some time, sound becomes a happening that passes through you as opposed to making a dense impact.

Anapanasati is not something easy because breath sensations calm with awareness upon them; often making them difficult to discern.

Often, a hindrance to anapanasati is the very intention to watch or focus on the breathing because intention itself is also a thought that diminishes the awareness of the mind.

Its like "listening" to another person talking. If you really want to really hear what the other person is saying, you must abandon thoughts and have an open quiet mind.

If the mind can abide without thoughts; then the meditator should also abandon the intention to "focus" on breathing. If the mind is quiet & silent yet also soft & fluid, it will naturally flow into the breathing; similar to how water naturally flows into cracks and crevices.

The Tao Te Ching Verse 8 says:

The highest good is like water. Water give life to the ten thousand things and does not strive. It flows in places men reject and so is like the Tao.

The 3rd Zen Patriarch said:

The Supreme Way is not difficult If only you do not pick and choose. Neither love nor hate, And you will clearly understand. Be off by a hair, And you are as far from it as heaven from earth.

If you want the Way to appear, Be neither for nor against. For and against opposing each other This is the mind's disease. Without recognizing the mysterious principle It is useless to practice quietude.

The Way is perfect like great space, Without lack, without excess. Because of grasping and rejecting, You cannot attain it.

Hsin Hsin Ming

Welcome to the in-betweens! When someone starts meditating, there are generally a few phase that they will pass through. These are all well documented, though I personally think they find the best expression in the 10 Ox herding pictures from Zen. The first is quieting the mind. In the series of Ox herding pictures, this is the state of seeing and taming the ox where the mind's unruly nature is first seen for what it is it's thrashings are subdued:

Once the mind settles, however, there is a very lengthy process of riding the ox to the mountain:

This seems to be where you are at now. Take special note of that second painting. I selected this artist's rendition specifically because I think it beautifully characterizes this phase of training. The adept is riding the ox backwards. He literally has no idea where he's going. Indeed, this stage of training is very much categorized by a feeling of bumping around in the dark. And it can go on for years. Or it can be over in a matter of days. There is literally no way of knowing how long it will take. The adept doesn't even know if there is a mountain awaiting him! He has no idea what's ahead!

The only thing you can actively do at this stage is practice with sincerity. Sit daily for at least an hour. Apply your mind to your object of meditation. Don't worry about what is a distraction and what isn't. The mind is entirely too complicated to sort those things out at this stage. Moreover, much of what drives us into distraction is completely unconscious! As you go forward, you might even find your mind filled with thoughts again as deep, forgotten memories or unseen, habitual obstacles begin to rise to the surface. Lastly, I would strongly suggest that you go on retreats of three days or more. This will go a long way to loosening the hold those subtle, unseen, and unconscious distractions have on your mind. As they begin to fall away, your concentration will strengthen almost seemingly without effort.

If your practice has sincerity, eventually you will arrive at the foot of the mountain. You just have to persist.