I remember doing a whole book on the Sandokai a while back here titled
Branching Streams Flowing in the Dark. That was my in depth introduction to this poem, and this has been a nice refresher. Long before I got into Zen I had figured out the relativity of things to each other, how one can define the other and vice versa. I always thought it an interesting circle of thought, that there was a null quality to it because each also cancelled the other. If light/ri is defined by dark/ji, then there is no light/dark or ri/ji, or so I might get stuck if these were just thought experiments like I used to. The fun thing about Zen is that it's about the practice and not the thought experiment. If you sit zazen, like I did this morning, you begin to work with ri and ji, light and darkness, and if you sit zazen long enough you find yourself able to work with the relative and absolute in the reality of your life, at least to some degree. Now, I get shot by my own arrow points all the time, but that just shows that I keep shooting those arrows as I work on improving my aim.
To me, the whole point of the Sandokai and all the other vows covered in this book, and all the koans that I am happy to get back to, is that you have to take it out into the reality of your life. You have to find a way to practice it, because just thinking about it (or even non-thinking about it) alone does no meaningful good. If it ain't practical, then there ain't much point to the arrow at all.