What would you say to yourself if you knew you could say something to yourself, and the you that you knew was you would know that it was you, but the you that told you didn't know it would be you?
Helpful to think about it as if a person is always changing. So every instant there is a new you...
Like, for example, if you recieved a letter from yourself, but couldn't recall what was in that letter, or even writing it, or it ever even having existed, what would be in that letter?
Or more appropriately, what would you write in that letter now? Trusting, but not knowing, that another you would read it?
" Dear __________,
This is me. I know you can't possibly know that...because you don't ever remember writing this, but I can assure you that you did. You will have to trust me on that.
Anyway, I would just like to tell you that....."
Or, simply "What would your future self tell your present self?"
(Assuming your future self includes all of your future self, all of your past self, and all of present self)