Shikantaza was the very essence of life in Dogen’s monastery. The word he used for “monastery,” sorin, literally means a forest in which various kinds of trees are living together. In a monastery, all practitioners with their different characters, capabilities, and backgrounds live together with unified bodhi-mind; thus Zen monasteries are sometimes called sorin.
In Shobogenzo Ryugin, Dogen speaks of withered trees, and of mountain trees, ocean trees, and sky trees. ”Withered tree” is a common Buddhist metaphor for a monk who has, in meditation, attained a deep state of non-emotion, whose passions have all but disappeared. This meditative state is not to be confused with a quietistic or blissful condition, which is simply a passing phase that may arise in spiritual practice.
“Mountain trees” are those who are sitting as still as a mountain, while “ocean trees” are those who are exploring the great depths, and “sky trees” are those who are exploring the unbounded. All of these “trees” come together in a monastery, and the Zazen Hall where the mediation happens is sometimes called a koboku-dō, or “withered tree hall.”