Great Master Kassan Zenne (805–881) was a successor of Master Sensu Tokujō. At the suggestion of Master Dōgo Enchi, he visited Master Sensu and attained the truth under him. Later he lived and taught on Mount Kassan.
It is said that Master Jōzan Shinei, a successor of Master Isan Reiyū, once told Kassan,
“Because in life and death there is no buddha, then it is not life and death.” Kassan replied, “Because in life and death there is buddha, then we are not deluded by life and death.”
Although the words “life” and “death” exist in all languages, Zen Master Dōgen taught that we are not able to understand intellectually what our life and death actually are, saying that their real meaning is embedded in our actual day-to-day life itself. He described life and death as the real momentary state of the present moment. In our daily life, life and death both exist in undivided wholeness.
Dōgen paraphrased Kassan's exchange with Jōzan by writing,
"Because there is buddha within living and dying, life and death do not exist. We can also say: Because in life and death there is no buddha, we are not deluded by living and dying."
Dōgen uses the phrase "living and dying" to refer to the ever-flowing, ever-changing conditions that have no permanency, and the phrase "life and death" to refer to the delusion of static, unchanging conditions that are created by the discriminating mind. So, by substitution, we can say,
Because living and dying exists as ever-flowing, ever-changing conditions that have no permanency, life and death are not the static, unchanging conditions falsely perceived by the intellect. We can also say that since the static, unchanging "life and death" perceived by the intellect is a delusion, we should be present with the ever-flowing, ever-changing conditions of impermanent living and dying.
However we conceive of life and death, they are just that - concepts, mental constructions, schema, samskara. But real living and real dying are the actual experience of this very instant, this very moment, not the past of our memory or the future of our imagination. In this moment, right now, life neither comes into existence not disappears, it's just here, as it is, thus.
It is a mistake, Dōgen taught, to think that we go from being alive to being dead. In the Genjō-kōan, he said that although firewood becomes ash and can never go back to being firewood again, we should not take the view that ash is its future and firewood is its past. Remember, in the eternal here-and-now, firewood abides in the place of firewood and ash exists in the place of ash. Although each has a past and a future, the past and the future are cut off in the eternal now.
Similarly, human beings, after death, do not live again. At the same time, it is an established custom in Buddhism not to say that life turns into death. Instead, we speak of “no appearance.” How can something end that does not exist in the first place? And in the Buddha’s very earliest teachings, he established that death does not turn into life. This is why we speak of “no disappearance.” How can something exist that does not end?
Life, then, is an instantaneous situation, that is, the state of this very instant, and death is also an instantaneous situation.