Shikantaza has been defined as abiding in a state of brightly alert attention which is characterized by being free of thoughts, directed to no object, and attached to no particular content. This state of brightly alert attention is considered the purest form of zazen. ...
Dogen referred to shikantaza or zazen as total engagement in immobile sitting. This is not a passive state of meditation where we let ourselves be spaced out or half-asleep, but it’s also not controlling thoughts or holding them at bay, nor is it being absorbed in one object to the exclusion of everything else, such as being absorbed in a mantra, being absorbed in staring at something like a candle flame, or even being absorbed in the breath. One-pointed concentration on the breath which blocks thoughts and feelings is not Dogen’s zazen. The field of awareness in zazen is wide, open and inclusive – it includes awareness of sound, awareness of posture, of our muscles and any tension, and so on. Non-thinking is an active, dynamic engagement with our being, just as it is.
In the book Soto Zen, Shohaku Okumura compared non-thinking to a car engine that’s idling in neutral. Even though the engine is working, the gears aren’t engaged so the car doesn’t move. He said when we are thinking non-thinking, "We cannot say that there is no thinking. And we cannot say that we are thinking....Thoughts are simply idling." In zazen, the mind is alive and functioning but it isn’t actively, intentionally producing thoughts. When thoughts arise, if the mind is bright and alert, it is much easier to let them flow through without grabbing onto and developing them. Okumura said, "...by keeping an upright posture, without either rejecting or chasing after anything, we aren’t controlled by delusive thoughts." Non-thinking can only be understood by experiencing it through a non-discursive state such as zazen – so talking about it misses the point.
In Zen Mind, Beginner’s Mind, Suzuki Roshi taught that when we’re practicing zazen, we shouldn’t try to stop our thinking, rather, we should let it stop by itself. He said, "If something comes into your mind, let it come in, and let it go out....When you try to stop your thinking, it means you're are bothered by it. Do not be bothered by anything. It appears as if something comes from outside your mind, but actually it is only the waves of your mind, and if you are not bothered by the waves, gradually they will be come calmer and calmer."
The idea that enlightenment is not separate from our everyday activity is a characteristic of the Zen School. This doesn’t mean that whatever we happen to be doing or thinking is enlightened activity. It means that when we are able to collect our attention and bring our presence to what we are doing, we can experience our innate completeness, our wholeness of being. When we are awake to our present activity, then it is Buddha’s activity. Even if we are feeling angry or depressed, bored or uncomfortable, when we can surrender our resistance to it and completely accept our experience just as it is, without trying to change it or improve it, this undivided acceptance is undivided presence or Buddha-mind.