Attention involves seeing and hearing. We hear not only with our ears but also we are sensitive to the tones, the voice, to the implication of words, to hear without interference, to capture instantly the depth of a sound. Sound plays an extraordinary part in our lives: the sound of thunder, a flute playing in the distance, the unheard sound of the universe; the sound of silence, the sound of one's own heart beating; the sound of a bird and the noise of a man walking on the pavement; the waterfall. The universe is filled with sound. This sound has its own silence; all living things are involved in this sound of silence. To be attentive is to hear this silence and move with it.

Vol. II, p. 30 - Letters to the Schools (1981, 1985)