Regardless of his age or experience, a true teacher is simply one who has apprehended the true teaching and attained the authentic teacher’s seal of realization. He does not put texts first or understanding first, but his capacity is outside any framework and his spirit freely penetrates the nodes in bamboo. He is not concerned with self-views and does not stagnate in emotional feelings. Thus, practice and understanding are in mutual accord. This is a true master.
— Dogen, from Guidelines for Studying the Way