I’m not a scientist, but wouldn’t it make sense for standard practice to be to assume at first that there’s a shared variable (that you have introduced) and to

I’m not a scientist, but wouldn’t it make sense for standard practice to be to assume at first that there’s a shared variable (that you have introduced) and to look for it until you’re certain the things you’re plotting are independent? Of course they may not be in the end as that’s the “goal”, but the shared variable if there is indeed causation in that case will be what you’re looking for, not one of the variables you “know”.