Yet fish never once leave the waters, birds never forsake the sky. When their need is great, there is great activity. When their need is small, there is small activity.
In this way, none ever fails to exert itself to the full, and nowhere does any fail to move and turn freely. If a bird leaves the sky, it will soon die. If a fish leaves the water, it at once perishes.
We should grasp that water means life (for the fish), and the sky means life (for the bird). It must be that the bird means life (for the sky), and the fish means life (for the water); that life is the bird, life is the fish.
We could continue in this way even further, because practice and realization, and for all that is possessed of life, it is the same.
From The Heart of Dogen’s Shobogenzo, Suny Press, 2002
Norman Waddell and Masao Abe, translators