Dogen's Genjo Koan: Section Five

Firewood becomes ash, and it does not become firewood again. Yet, do not suppose that the ash is after and the firewood before. You should understand that firewood abides in the phenomenal expression of firewood, which fully includes before and after and is independent of before and after. Ash abides in the phenomenal expression of ash, which fully includes before and after. Just as firewood does not become firewood again after it is ash, you do not return to birth after death.

This being so, it is an established way in buddha-dharma to deny that birth turns into death. Accordingly, birth is understood as no-birth. It is an unshakable teaching in the Buddha's discourse that death does not turn into birth. Accordingly, death is understood as no-death.

Birth is an expression complete this moment. Death is an expression complete this moment. They are like winter and spring. You do not call winter the beginning of spring, nor summer the end of spring.

This analogy about firewood and ash is really pointing to the nature of human existence. It's sometimes interpreted to mean that Dogen taught that there was no such thing as post-mortem rebirth and initially I interpreted it this way too. However, I don't think this is correct. However, having said that, there are other important Zen masters such as the 6th Patriarch who do point to rebirth in other realms in terms of states of being in this life - psychological interpretations of rebirth are not just a modern phenomenon. This section is an introduction to Dogen's theory of Uji, 'Being-Time'.

Firewood becomes ash, and it does not become firewood again.
Change occurs only in one direction. In modern physics we have a concept of the 'arrow of time' and this corresponds loosley with that. This is change from the conventional perspective.

Yet, do not suppose that the ash is after and the firewood before.
Having said that, since entities do not have a self or identity that is continuous or carried forward through time, it is incorrect to say that one state changes into another. Before it burns firewood is just firewood. By the time it is ash, the firewood is already gone. The 'firewood' nature or identity is not preserved and carried forward within the ash - it is always only exactly what it actually is at a given moment. One thing does not change state, because there is no 'one thing' that continues from the before to the after. Existence is momentary. This corresponds to an understanding that could be expressed as 'only the present moment exists - the past and future are illusions'.

...fully includes before and after and is independent of before and after.
Each moment or state includes its past and future - the universal laws of conditionality (causality) are what allow things to be what they are at any given moment - and there is no phenomena other than those laws of conditionality. And yet, simultaneously each moment or state is completely just itself, independent of it's past and future, because no self is carried forward through the change - from the before to the after.

Just as firewood does not become firewood again after it is ash, you do not return to birth after death.
This is the line that is perhaps most tempting to interpret as a denial of rebirth. But (as I recall, please correct me otherwise) Dogen makes reference to literal rebirth elsewhere in his writing, so this can be taken as a reiteration that there is no self which is carried forward from one life to another. As one state never returns to its previous state, death never turns into life. That is, no self is ever carried forward to be reborn.

it is an established way in buddha-dharma to deny that birth turns into death
Yet, in Buddhism it is taught that life does not change into death. Because there are no selves, nothing is ever born, nothing really comes into being. In this sense there is no birth. It is also taught that death does not turn into life. Nothing is carried forward through death into the next life. In this sense there is no death. Since there are never any substantial selves, nothing ever comes into being or is destroyed. What we commonly see as birth and death is ultimately no birth and no death, that is The Unborn.

Birth is an expression complete this moment. Death is an expression complete this moment.
The Unborn isn't something that exists in addition to phenomena, it is phenomena just as they are. Things are always just as they are, and without the continuity of a real self to unite them, each state or moment is just itself, one does not become the other.