So when we practice zazen, all that exists is the movement of the breathing, but we are aware of this movement. You should not be absent-minded. But to be aware of the movement does not mean to be aware of your small self, but rather of your universal nature, or Buddha nature. This kind of awareness is very important, because we are usually so one-sided.
Our usual understanding of life is dualistic: you and I, this and that, good and bad. But actually these discriminations are themselves the awareness of the universal existence. “You” means to be aware of the universe in the form of you, and “I” means to be aware of it in the form of I. You and I are just swinging doors. This kind of understanding is necessary. This should not even be called understanding; it is actually the true experience of life through Zen practice.
From Zen Mind, Beginner’s Mind by Shunryu Suzuki, page 12