#but I was washing dishes and this idea struck me like a metal pole

[sarshles-cheescake-li]

I wonder if Lu Guang and Cheng Xiaoshi's rule around past and future was about protecting the timeline and the future, or about protecting Cheng Xiaoshi?

After all, on a universal, metaphysical scale of things, physics shouldn't care about whether a change to its pathway is observable to humans or not. It shouldn't care whether someone changes the timeline enough to have noticeable changes on the human level, or just on the micro level -- one second late here, a few centimetres too far here.

Besides, I would presume that for the sake of the universe's peace of mind, it wouldn't give people reality-breaking powers for the fun of it.

But what we do know is that changing the past drains you. It tires you out, wrings the smile from your face and the crinkle from your eyes. After all, if someone got murdered, to you, it's a tragedy. If someone got murdered because of something you did or didn't do, then to you, it's sin. And if they weren't supposed to die in the first place, but you went back and changed something, and now they are? That's practically your crime, isn't it (to clarify, this is a literary hyperbole)?

But you can't just not change the past. Because if someone gets hurt and you could've saved them, then that's no different, really, from dooming them yourself. Does inaction not carry the same consequences as action? Does the crime of omission not weigh on the scale just as heavily as the crime of commission?

Cheng Xiaoshi learns. Cheng Xiaoshi cares, about everyone and everything, deeply. Whether it be because he dives in and inherits their emotions, or because he is too full of love for every single living thing, Cheng Xiaoshi is the type of person to see everyone whom he can reach as his responsibility. But that is a horrible, horrible mindset for a time-traveler, isn't it?

So -- imagine. Lu Guang, in the first few loops, without his rules, changing the past with his partner, watching Cheng Xiaoshi fall apart again and again, drowned by his own perceived sins. And we don't know who Lu Guang was, at this point. Was he perhaps more stilted, less able to be Cheng Xiaoshi's psychologist, as director Li once joked? What would've happened to a Cheng Xiaoshi who carried more burdens and whose Lu Guang was not yet practiced at bringing him back up from his lowest points?

So Lu Guang learns. The first rule is a time limit, and the second rule of obedience. Maybe this was their own rule to begin with, to make sure Cheng Xiaoshi remained within Lu Guang's reach. Maybe it was made later, so that Lu Guang could guide Cheng Xiaoshi and shoulder more responsibility. If he could make himself the gunman and Cheng Xiaoshi the gun, maybe the latter would not feel so guilty over whom gets caught in the crossfire.

Maybe later he learns that that doesn't work, that Cheng Xiaoshi still feels deeply guilty about the people hurt by the new histories they create. So then the third rule: Past or future, leave them be. I had always found it strange why this rule included the future, in addition to the past. If it's about timeline stability, wouldn't it just be about the past?

But maybe it's for Cheng Xiaoshi's sake. If Cheng Xiaoshi thinks that he isn't allowed to change the past for some higher, nobler reason, if it's Lu Guang making him follow the rules and not he himself, then the choice is out of his hands. It's not that he's "leaving someone to die." It's the timeline which demands they must die. He cannot change it, so he will not. And if it's for that same higher, nobler reason that he cannot try to track the changes that he created, then that choice is out of his hands, too. Then it's about him "not wanting to know what happens" it's that he "shouldn't know what happens."

Cheng Xiaoshi, left to its own devices, flies too high and burns himself out, trying to care for too many people too deeply, all at once. But what if Lu Guang chains him to the ground? Here are the rules, Cheng Xiaoshi. You must stay for only twelve hours (So I will know where you are and you will not lose yourself living infinitely in the past). You must listen to me (so you can punch me and blame me if things go wrong instead of yourself). You cannot change the past (So please don't try to, because once you do, it will suffocate you) and you cannot ask about the future (you don't need to know what your actions have wrought). And Cheng Xiaoshi remains, unburnt, unscathed. If anyone asks him why he won't fly higher, it's a simple answer -- Lu Guang asked me not to.

Lastly, this one is purely speculation, and I don't think it's true, but -- do we actually get confirmation, at any point in the story, that Cheng Xiaoshi can't dive into photos that he's already dove into? Did he ever actually try and have it fail, or did Lu Guang simply tell him that?

Maybe there is no diving limit. Maybe Cheng Xiaoshi can dive into the same photo as many times as he wants. But then why would Lu Guang tell he couldn't?

It's simple.

You must not get caught up trying to change a past that doesn't want to be changed.

After all, Lu Guang knows very well what happens if you do, doesn't he?