Here is the quote mentioned this morning from Shido Bunan Zenji (1603-1676, who was Hakuin’s teacher’s teacher).
He often told his students, “There is no special doctrine for the study of Zen. All that is needed is to see it directly, hear it directly. In direct seeing there is no seeing. In direct hearing there is no hearing. This is possible when you naturally become one piece, with no in and no out.”